diff --git a/src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.Cookies.cs b/src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.Cookies.cs
index 714cec7bae3d56..6ecd7261ca2d97 100644
--- a/src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.Cookies.cs
+++ b/src/libraries/Common/tests/System/Net/Http/HttpClientHandlerTest.Cookies.cs
@@ -44,7 +44,7 @@ private static CookieContainer CreateSingleCookieContainer(Uri uri, string cooki
         private static string GetCookieHeaderValue(string cookieName, string cookieValue) => $"{cookieName}={cookieValue}";
 
         [Fact]
-        public async Task GetAsync_DefaultCoookieContainer_NoCookieSent()
+        public virtual async Task GetAsync_DefaultCoookieContainer_NoCookieSent()
         {
             await LoopbackServerFactory.CreateClientAndServerAsync(
                 async uri =>
diff --git a/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.AltSvc.cs b/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.AltSvc.cs
index d20b0f2635dcab..d6a53a33e2342e 100644
--- a/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.AltSvc.cs
+++ b/src/libraries/System.Net.Http/tests/FunctionalTests/HttpClientHandlerTest.AltSvc.cs
@@ -29,10 +29,15 @@ private HttpClient CreateHttpClient(Version version)
             return client;
         }
 
-        [Theory]
+        [ConditionalTheory]
         [MemberData(nameof(AltSvcHeaderUpgradeVersions))]
         public async Task AltSvc_Header_Upgrade_Success(Version fromVersion, bool overrideHost)
         {
+            if (UseVersion == HttpVersion30 && fromVersion == HttpVersion.Version11 && overrideHost)
+            {
+                throw new SkipTestException("https://github.com/dotnet/runtime/issues/91757");
+            }
+
             // The test makes a request to a HTTP/1 or HTTP/2 server first, which supplies an Alt-Svc header pointing to the second server.
             using GenericLoopbackServer firstServer =
                 fromVersion.Major switch
diff --git a/src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs b/src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs
index 67604ca426e798..f91fc2dda1dd3c 100644
--- a/src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs
+++ b/src/libraries/System.Net.Http/tests/FunctionalTests/SocketsHttpHandlerTest.cs
@@ -1633,6 +1633,7 @@ public SocketsHttpHandler_HttpClientHandler_MaxResponseHeadersLength_Http2(ITest
         protected override Version UseVersion => HttpVersion.Version20;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     [ActiveIssue("https://github.com/dotnet/runtime/issues/91757")]
     [ActiveIssue("https://github.com/dotnet/runtime/issues/101015")]
@@ -4027,6 +4028,7 @@ public SocketsHttpHandler_HttpClientHandler_Cancellation_Test_Http2(ITestOutputH
         protected override Version UseVersion => HttpVersion.Version20;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandlerTest_HttpClientHandlerTest_Http3 : HttpClientHandlerTest
     {
@@ -4034,13 +4036,19 @@ public SocketsHttpHandlerTest_HttpClientHandlerTest_Http3(ITestOutputHelper outp
         protected override Version UseVersion => HttpVersion.Version30;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandlerTest_Cookies_Http3 : HttpClientHandlerTest_Cookies
     {
         public SocketsHttpHandlerTest_Cookies_Http3(ITestOutputHelper output) : base(output) { }
         protected override Version UseVersion => HttpVersion.Version30;
+
+        [Fact]
+        [ActiveIssue("https://github.com/dotnet/runtime/issues/91757")]
+        public override Task GetAsync_DefaultCoookieContainer_NoCookieSent() { return null!; }
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandlerTest_HttpClientHandlerTest_Headers_Http3 : HttpClientHandlerTest_Headers
     {
@@ -4048,6 +4056,7 @@ public SocketsHttpHandlerTest_HttpClientHandlerTest_Headers_Http3(ITestOutputHel
         protected override Version UseVersion => HttpVersion.Version30;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandler_HttpClientHandler_Cancellation_Test_Http3 : SocketsHttpHandler_Cancellation_Test
     {
@@ -4055,6 +4064,7 @@ public SocketsHttpHandler_HttpClientHandler_Cancellation_Test_Http3(ITestOutputH
         protected override Version UseVersion => HttpVersion.Version30;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandler_HttpClientHandler_AltSvc_Test_Http3 : HttpClientHandler_AltSvc_Test
     {
@@ -4062,6 +4072,7 @@ public SocketsHttpHandler_HttpClientHandler_AltSvc_Test_Http3(ITestOutputHelper
         protected override Version UseVersion => HttpVersion.Version30;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandler_HttpClientHandler_Finalization_Http3 : HttpClientHandler_Finalization_Test
     {
@@ -4226,6 +4237,7 @@ public SocketsHttpHandler_RequestContentLengthMismatchTest_Http2(ITestOutputHelp
         protected override Version UseVersion => HttpVersion.Version20;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandler_RequestContentLengthMismatchTest_Http3 : SocketsHttpHandler_RequestContentLengthMismatchTest
     {
@@ -4402,6 +4414,7 @@ public SocketsHttpHandler_SocketsHttpHandler_SecurityTest_Http2(ITestOutputHelpe
         protected override Version UseVersion => HttpVersion.Version20;
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandler_SocketsHttpHandler_SecurityTest_Http3 : SocketsHttpHandler_SecurityTest
     {
@@ -4530,6 +4543,7 @@ await Http11LoopbackServerFactory.Singleton.CreateClientAndServerAsync(async uri
         }
     }
 
+    [Collection(nameof(DisableParallelization))]
     [ConditionalClass(typeof(HttpClientHandlerTestBase), nameof(IsQuicSupported))]
     public sealed class SocketsHttpHandler_HttpRequestErrorTest_Http30 : SocketsHttpHandler_HttpRequestErrorTest
     {
diff --git a/src/libraries/System.Net.Quic/tests/FunctionalTests/QuicTestCollection.cs b/src/libraries/System.Net.Quic/tests/FunctionalTests/QuicTestCollection.cs
index 5125c72e0ca8d1..f8dd160acb00b7 100644
--- a/src/libraries/System.Net.Quic/tests/FunctionalTests/QuicTestCollection.cs
+++ b/src/libraries/System.Net.Quic/tests/FunctionalTests/QuicTestCollection.cs
@@ -15,7 +15,7 @@
 
 namespace System.Net.Quic.Tests;
 
-[CollectionDefinition(nameof(QuicTestCollection))]
+[CollectionDefinition(nameof(QuicTestCollection), DisableParallelization = true)]
 public unsafe class QuicTestCollection : ICollectionFixture<QuicTestCollection>, IDisposable
 {
     public static bool IsSupported => QuicListener.IsSupported && QuicConnection.IsSupported;