diff --git a/tox.ini b/tox.ini
index da2a1f1..776a5f2 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,7 +1,10 @@
 [tox]
 envlist = pep8,py27,py34,py35
 skipsdist = True
-skip_missing_interpreters = True
+# NOTE(beisner): Avoid build/test env pollution by not enabling sitepackages.
+sitepackages = False
+# NOTE(beisner): Avoid false positives by not skipping missing interpreters.
+skip_missing_interpreters = False
 
 [testenv]
 setenv = VIRTUAL_ENV={envdir}