diff --git a/prebuilt.py b/prebuilt.py
index cf03157a9f..d14c801dd0 100755
--- a/prebuilt.py
+++ b/prebuilt.py
@@ -9,6 +9,7 @@ import optparse
 import os
 import re
 import sys
+import tempfile
 
 from chromite.lib import cros_build_lib
 """
@@ -190,6 +191,63 @@ def ShouldFilterPackage(file_path):
   return False
 
 
+def _ShouldFilterPackageFileSection(section):
+  """Return whether an section in the package file should be filtered out.
+
+  Args:
+    section: The section, as a list of strings.
+
+  Returns:
+    True if the section should be excluded.
+  """
+
+  for line in section:
+    if line.startswith("CPV: "):
+      package = line.replace("CPV: ", "").rstrip()
+      if ShouldFilterPackage(package):
+        return True
+  else:
+    return False
+
+
+def FilterPackagesFile(packages_filename):
+  """Read a portage Packages file and filter out private packages.
+
+  The new, filtered packages file is written to a temporary file.
+
+  Args:
+    packages_filename: The filename of the Packages file.
+
+  Returns:
+    filtered_packages: A filtered Packages file, as a NamedTemporaryFile.
+  """
+
+  packages_file = open(packages_filename)
+  filtered_packages = tempfile.NamedTemporaryFile()
+  section = []
+  for line in packages_file:
+    if line == "\n":
+      if not _ShouldFilterPackageFileSection(section):
+        # Looks like this section doesn't contain a private package. Write it
+        # out.
+        filtered_packages.write("".join(section))
+
+      # Start next section.
+      section = []
+
+    section.append(line)
+  else:
+    if not _ShouldFilterPackageFileSection(section):
+      filtered_packages.write("".join(section))
+  packages_file.close()
+
+  # Flush contents to disk.
+  filtered_packages.flush()
+  filtered_packages.seek(0)
+
+  return filtered_packages
+
+
 def _GsUpload(args):
   """Upload to GS bucket.
 
@@ -203,6 +261,10 @@ def _GsUpload(args):
   if ShouldFilterPackage(local_file):
     return
 
+  if local_file.endswith("/Packages"):
+    filtered_packages_file = FilterPackagesFile(local_file)
+    local_file = filtered_packages_file.name
+
   cmd = '%s cp -a public-read %s %s' % (_GSUTIL_BIN, local_file, remote_file)
   # TODO(scottz): port to use _Run or similar when it is available in
   # cros_build_lib.
diff --git a/prebuilt_unittest.py b/prebuilt_unittest.py
index 29a02faffd..4fe0c91cc9 100755
--- a/prebuilt_unittest.py
+++ b/prebuilt_unittest.py
@@ -191,5 +191,41 @@ class TestPrebuilt(unittest.TestCase):
                       'asdfasdf')
 
 
+class TestPackagesFileFiltering(unittest.TestCase):
+
+  def setUp(self):
+    self.mox = mox.Mox()
+
+  def tearDown(self):
+    self.mox.UnsetStubs()
+    self.mox.VerifyAll()
+
+  def testFilterAllPackages(self):
+    self.mox.StubOutWithMock(prebuilt, 'ShouldFilterPackage')
+    prebuilt.ShouldFilterPackage("public1").AndReturn(False)
+    prebuilt.ShouldFilterPackage("private").AndReturn(True)
+    prebuilt.ShouldFilterPackage("public2").AndReturn(False)
+    full_packages_file = [
+      "foo: bar\n", "\n",
+      "CPV: public1\n", "foo: bar1\n", "\n",
+      "CPV: private\n", "foo: bar2\n", "\n",
+      "CPV: public2\n", "foo: bar3\n", "\n",
+    ]
+    private_packages_file = [
+      "foo: bar\n", "\n",
+      "CPV: public1\n", "foo: bar1\n", "\n",
+      "CPV: public2\n", "foo: bar3\n", "\n",
+    ]
+    self.mox.ReplayAll()
+    temp_packages_file = tempfile.NamedTemporaryFile()
+    temp_packages_file.write("".join(full_packages_file))
+    temp_packages_file.flush()
+    new_packages_file = prebuilt.FilterPackagesFile(temp_packages_file.name)
+    new_contents = open(new_packages_file.name).read()
+    self.assertEqual("".join(private_packages_file), new_contents)
+    self.assertEqual("".join(private_packages_file), new_packages_file.read())
+    new_packages_file.close()
+
+
 if __name__ == '__main__':
   unittest.main()