diff --git a/bootstrap.py b/bootstrap.py
index 1b28969ae92..6b7e45ccac0 100644
--- a/bootstrap.py
+++ b/bootstrap.py
@@ -56,6 +56,9 @@
                         "file to be used."))
 parser.add_option("-f", "--find-links",
                   help=("Specify a URL to search for buildout releases"))
+parser.add_option("--allow-site-packages",
+                  action="store_true", default=False,
+                  help=("Let bootstrap.py use existing site packages"))
 
 
 options, args = parser.parse_args()
@@ -63,32 +66,38 @@
 ######################################################################
 # load/install setuptools
 
-to_reload = False
 try:
-    import pkg_resources
-    import setuptools
+    if options.allow_site_packages:
+        import setuptools
+        import pkg_resources
+    from urllib.request import urlopen
 except ImportError:
-    ez = {}
-
-    try:
-        from urllib.request import urlopen
-    except ImportError:
-        from urllib2 import urlopen
-
-    # XXX use a more permanent ez_setup.py URL when available.
-    exec(urlopen('https://bitbucket.org/pypa/setuptools/raw/0.7.2/ez_setup.py'
-                ).read(), ez)
-    setup_args = dict(to_dir=tmpeggs, download_delay=0)
-    ez['use_setuptools'](**setup_args)
-
-    if to_reload:
-        reload(pkg_resources)
-    import pkg_resources
-    # This does not (always?) update the default working set.  We will
-    # do it.
-    for path in sys.path:
-        if path not in pkg_resources.working_set.entries:
-            pkg_resources.working_set.add_entry(path)
+    from urllib2 import urlopen
+
+ez = {}
+exec(urlopen('https://bitbucket.org/pypa/setuptools/downloads/ez_setup.py'
+            ).read(), ez)
+if not options.allow_site_packages:
+    # ez_setup imports site, which adds site packages
+    # this will remove them from the path to ensure that incompatible versions 
+    # of setuptools are not in the path
+    import site
+    # inside a virtualenv, there is no 'getsitepackages'. 
+    # We can't remove these reliably
+    if hasattr(site, 'getsitepackages'):
+        for sitepackage_path in site.getsitepackages():
+            sys.path[:] = [x for x in sys.path if sitepackage_path not in x]
+
+setup_args = dict(to_dir=tmpeggs, download_delay=0)
+ez['use_setuptools'](**setup_args)
+import setuptools
+import pkg_resources
+
+# This does not (always?) update the default working set.  We will
+# do it.
+for path in sys.path:
+    if path not in pkg_resources.working_set.entries:
+        pkg_resources.working_set.add_entry(path)
 
 ######################################################################
 # Install buildout