diff --git a/.travis.yml b/.travis.yml
index 91985fe..4176898 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,4 +3,7 @@ services:
 - docker
 script:
 - docker build -t google/python .
-- docker build -t google/python-libraries tests/python2-libraries
+- docker build tests/virtualenv
+- docker build tests/no-virtualenv
+# Disabled temporarily.
+#- docker build -t google/python-libraries tests/python2-libraries
diff --git a/Dockerfile b/Dockerfile
index 38fa366..959814b 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -22,12 +22,12 @@ ENV LANG C.UTF-8
 # install virtualenv system-wide.
 RUN pip install --upgrade pip virtualenv
 
-EXPOSE 8080
-
 RUN ln -s /home/vmagent/app /app
 WORKDIR /app
 
+# Port 8080 is the port used by Google App Engine for serving HTTP traffic.
+EXPOSE 8080
 ENV PORT 8080
 
-CMD []
 # The user's Dockerfile must specify an entrypoint with ENTRYPOINT or CMD.
+CMD []
diff --git a/README.md b/README.md
index 10c41cd..c97c6bf 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@ For other docker hosts, you'll need to create a `Dockerfile` based on this image
     RUN virtualenv /env
     
     # Setting these environment variables are the same as running
-    # source /env/bin/activate
+    # source /env/bin/activate.
     ENV VIRTUAL_ENV /env
     ENV PATH /env/bin:$PATH
     
diff --git a/tests/no-virtualenv/Dockerfile b/tests/no-virtualenv/Dockerfile
new file mode 100644
index 0000000..68089c8
--- /dev/null
+++ b/tests/no-virtualenv/Dockerfile
@@ -0,0 +1,5 @@
+FROM google/python
+
+RUN if [ "$(which python)" != "/usr/bin/python" ]; then exit 1; fi;
+RUN pip install gunicorn
+RUN if [ "$(which gunicorn)" != "/usr/local/bin/gunicorn" ]; then exit 1; fi;
diff --git a/tests/virtualenv/Dockerfile b/tests/virtualenv/Dockerfile
new file mode 100644
index 0000000..d4effb5
--- /dev/null
+++ b/tests/virtualenv/Dockerfile
@@ -0,0 +1,28 @@
+FROM google/python
+
+ENV VIRTUAL_ENV /env
+ENV PATH /env/bin:$PATH
+
+RUN if [ "$(which python)" != "/usr/bin/python" ]; then exit 1; fi;
+RUN virtualenv /env
+# All commands from this point on should use the virtualenv
+RUN if [ "$(which python)" != "/env/bin/python" ]; then exit 1; fi;
+RUN if [ "$(python --version 2>&1)" != "Python 2.7.9" ]; then exit 1; fi;
+RUN if [ "$(which pip)" != "/env/bin/pip" ]; then exit 1; fi;
+RUN pip install gunicorn flask
+RUN if [ "$(which gunicorn)" != "/env/bin/gunicorn" ]; then exit 1; fi;
+RUN python -c "import sys; import flask; sys.exit(0 if flask.__file__.startswith('/env') else 1)"
+
+# Python 3
+RUN rm -rf /env
+RUN if [ "$(which python3.4)" != "/usr/bin/python3.4" ]; then exit 1; fi;
+RUN virtualenv -p python3.4 /env
+# All commands from this point on should use the virtualenv
+RUN if [ "$(which python)" != "/env/bin/python" ]; then exit 1; fi;
+RUN if [ "$(which python3)" != "/env/bin/python3" ]; then exit 1; fi;
+RUN if [ "$(python --version 2>&1)" != "Python 3.4.2" ]; then exit 1; fi;
+RUN if [ "$(which pip)" != "/env/bin/pip" ]; then exit 1; fi;
+RUN if [ "$(which pip3)" != "/env/bin/pip3" ]; then exit 1; fi;
+RUN pip install gunicorn flask
+RUN if [ "$(which gunicorn)" != "/env/bin/gunicorn" ]; then exit 1; fi;
+RUN python -c "import sys; import flask; sys.exit(0 if flask.__file__.startswith('/env') else 1)"