diff --git a/test/runtests.jl b/test/runtests.jl
index 2f9cd058bb..d8d6d15b8c 100644
--- a/test/runtests.jl
+++ b/test/runtests.jl
@@ -83,8 +83,11 @@ prepend!(tests, linalg_tests)
 import LinearAlgebra
 cd(@__DIR__) do
     n = 1
-    if net_on
-        n = min(Sys.CPU_THREADS, length(tests))
+    # Allow parallel tests with isolated environment
+    # https://github.com/JuliaLang/julia/issues/43205
+    if net_on || haskey(ENV, "JULIA_CPU_THREADS")
+        x = haskey(ENV, "JULIA_CPU_THREADS") ? parse(Int, ENV["JULIA_CPU_THREADS"]) : Sys.CPU_THREADS
+        n = min(x, length(tests))
         n > 1 && addprocs_with_testenv(n)
         LinearAlgebra.BLAS.set_num_threads(1)
     end