diff --git a/Library/Homebrew/test/test_external_deps.rb b/Library/Homebrew/test/test_external_deps.rb
new file mode 100644
index 0000000000..373d334ba7
--- /dev/null
+++ b/Library/Homebrew/test/test_external_deps.rb
@@ -0,0 +1,120 @@
+require 'testing_env'
+
+require 'extend/ARGV' # needs to be after test/unit to avoid conflict with OptionsParser
+ARGV.extend(HomebrewArgvExtension)
+
+require 'test/testball'
+require 'formula_installer'
+require 'utils'
+
+
+# Custom formula installer that checks deps but does not
+# run the install code. We also override the external dep
+# install messages, so for instance the Python check doesnt
+# look for the pip formula (which isn't avaialble in test
+# mode.)
+class DontActuallyInstall < FormulaInstaller
+  def install_private f ; end
+  
+  def pyerr dep
+    "Python module install message."
+  end
+  
+  def plerr dep
+    "Perl module install message."
+  end
+  
+  def rberr dep
+    "Ruby module install message."
+  end
+end
+
+
+
+class BadPerlBall <TestBall
+  depends_on "notapackage" => :perl
+
+  def initialize name=nil
+    super "uses_perl_ball"
+  end
+end
+
+class GoodPerlBall <TestBall
+  depends_on "ENV" => :perl
+
+  def initialize name=nil
+    super "uses_perl_ball"
+  end
+end
+
+class BadPythonBall <TestBall
+  depends_on "notapackage" => :python
+
+  def initialize name=nil
+    super "uses_python_ball"
+  end
+end
+
+class GoodPythonBall <TestBall
+  depends_on "datetime" => :python
+
+  def initialize name=nil
+    super "uses_python_ball"
+  end
+end
+
+class BadRubyBall <TestBall
+  depends_on "notapackage" => :ruby
+
+  def initialize name=nil
+    super "uses_ruby_ball"
+  end
+end
+
+class GoodRubyBall <TestBall
+  depends_on "date" => :ruby
+
+  def initialize name=nil
+    super "uses_ruby_ball"
+  end
+end
+
+
+class ExternalDepsTests < Test::Unit::TestCase
+  def check_deps_fail f
+    assert_raises(RuntimeError) do
+      DontActuallyInstall.new.install f.new
+    end
+  end
+
+  def check_deps_pass f
+    assert_nothing_raised do
+      DontActuallyInstall.new.install f.new
+    end
+  end
+
+
+  def test_bad_perl_deps
+    check_deps_fail BadPerlBall
+  end
+
+  def test_good_perl_deps
+    check_deps_pass GoodPerlBall
+  end
+
+  def test_bad_python_deps
+    check_deps_fail BadPythonBall
+  end
+
+  def test_good_python_deps
+    check_deps_pass GoodPythonBall
+  end
+
+  def test_bad_ruby_deps
+    check_deps_fail BadRubyBall
+  end
+
+  def test_good_ruby_deps
+    check_deps_pass GoodRubyBall
+  end
+end