diff --git a/nselib/ipOps.lua b/nselib/ipOps.lua
index 1279b8fac..4220e2370 100644
--- a/nselib/ipOps.lua
+++ b/nselib/ipOps.lua
@@ -655,6 +655,11 @@ hex_to_bin = function( hex )
 
 end
 
+--Ignore the rest if we are not testing.
+if not unittest.testing() then
+  return _ENV
+end
+
 test_suite = unittest.TestSuite:new()
 test_suite:add_test(unittest.is_true(isPrivate("192.168.123.123")), "192.168.123.123 is private")
 test_suite:add_test(unittest.is_false(isPrivate("1.1.1.1")), "1.1.1.1 is not private")
diff --git a/nselib/smbauth.lua b/nselib/smbauth.lua
index 9531e6766..78b6283a1 100644
--- a/nselib/smbauth.lua
+++ b/nselib/smbauth.lua
@@ -920,6 +920,10 @@ function calculate_signature(mac_key, data)
   end
 end
 
+if not unittest.testing() then
+  return _ENV
+end
+
 test_suite = unittest.TestSuite:new()
 test_suite:add_test(unittest.equal(
     stdnse.tohex(select(-1, lm_create_hash("passphrase"))),
diff --git a/nselib/unicode.lua b/nselib/unicode.lua
index a365d30fa..af43f0051 100644
--- a/nselib/unicode.lua
+++ b/nselib/unicode.lua
@@ -396,6 +396,10 @@ function utf8to16(from)
   return table.concat(buf)
 end
 
+if not unittest.testing() then
+  return _ENV
+end
+
 test_suite = unittest.TestSuite:new()
 test_suite:add_test(function()
     local pos, cp = utf8_dec("\xE6\x97\xA5\xE6\x9C\xAC\xE8\xAA\x9E")
diff --git a/nselib/unittest.lua b/nselib/unittest.lua
index c686c1fba..0bd2187ae 100644
--- a/nselib/unittest.lua
+++ b/nselib/unittest.lua
@@ -15,6 +15,9 @@
 --
 -- The library is driven by the unittest NSE script.
 --
+-- @args unittest.run Run tests. Causes <code>unittest.testing()</code> to
+--                    return true.
+--
 -- @copyright Same as Nmap--See http://nmap.org/book/man-legal.html
 
 local stdnse = require "stdnse"
@@ -136,11 +139,22 @@ local libs = {
 "xmpp",
 }
 
+local am_testing = stdnse.get_script_args('unittest.run')
+---Check whether tests are being run
+--
+-- Libraries can use this function to avoid the overhead of creating tests if
+-- the user hasn't chosen to run them.
+-- @return true if unittests are being run, false otherwise.
+function testing()
+  return am_testing
+end
+
 ---
 -- Run tests provided by NSE libraries
 -- @param to_test A list (table) of libraries to test. If none is provided, all
 --                libraries are tested.
 run_tests = function(to_test)
+  am_testing = true
   if to_test == nil then
     to_test = libs
   end
@@ -354,6 +368,11 @@ expected_failure = function(test)
   end
 end
 
+
+if not testing() then
+  return _ENV
+end
+
 -- Self test
 test_suite = TestSuite:new()
 
diff --git a/scripts/unittest.nse b/scripts/unittest.nse
index aa525f18d..242e79ba1 100644
--- a/scripts/unittest.nse
+++ b/scripts/unittest.nse
@@ -6,7 +6,6 @@ Runs unit tests on all NSE libraries.
 ]]
 
 ---
--- @args unittest.run Run tests
 -- @args unittest.tests Run tests from only these libraries (defaults to all)
 --
 -- @usage
@@ -24,7 +23,7 @@ license = "Same as Nmap--See http://nmap.org/book/man-legal.html"
 categories = {"safe"}
 
 
-prerule = function() return stdnse.get_script_args("unittest.run") end
+prerule = unittest.testing
 
 action = function()
   local libs = stdnse.get_script_args("unittest.tests")