diff --git a/tox.ini b/tox.ini
index 525311a..e925450 100644
--- a/tox.ini
+++ b/tox.ini
@@ -17,11 +17,6 @@ passenv =
     # https://github.com/JuliaLang/julia/issues/18693
     SSL_CERT_FILE
 
-    # PyCall uses $PYTHON to configure Python executable.  It is
-    # useful to specify this in CI so that `diffeqpy.install()` can
-    # directly configure PyCall.
-    PYTHON
-
     JULIA_*
 
 [testenv:py3-numba]