diff --git a/wp-includes/link-template.php b/wp-includes/link-template.php
index 6e7747e9ba..fdc91e8937 100644
--- a/wp-includes/link-template.php
+++ b/wp-includes/link-template.php
@@ -1775,14 +1775,19 @@ function content_url($path = '') {
 */
 function plugins_url($path = '', $plugin = '') {
 	$scheme = ( is_ssl() ? 'https' : 'http' );
-	$url = WP_PLUGIN_URL;
+
+	if ( $plugin !== '' && preg_match('#^' . preg_quote(WPMU_PLUGIN_DIR . DIRECTORY_SEPARATOR, '#') . '#', $plugin) ) {
+		$url = WPMU_PLUGIN_URL;
+	} else {
+		$url = WP_PLUGIN_URL;
+	}
+
 	if ( 0 === strpos($url, 'http') ) {
 		if ( is_ssl() )
 			$url = str_replace( 'http://', "{$scheme}://", $url );
 	}
 
-	if ( !empty($plugin) && is_string($plugin) )
-	{
+	if ( !empty($plugin) && is_string($plugin) ) {
 		$folder = dirname(plugin_basename($plugin));
 		if ('.' != $folder)
 			$url .= '/' . ltrim($folder, '/');
diff --git a/wp-includes/plugin.php b/wp-includes/plugin.php
index 7b3ae86fbe..99169861c9 100644
--- a/wp-includes/plugin.php
+++ b/wp-includes/plugin.php
@@ -492,7 +492,9 @@ function plugin_basename($file) {
 	$file = preg_replace('|/+|','/', $file); // remove any duplicate slash
 	$plugin_dir = str_replace('\\','/',WP_PLUGIN_DIR); // sanitize for Win32 installs
 	$plugin_dir = preg_replace('|/+|','/', $plugin_dir); // remove any duplicate slash
-	$file = preg_replace('|^' . preg_quote($plugin_dir, '|') . '/|','',$file); // get relative path from plugins dir
+	$mu_plugin_dir = str_replace('\\','/',WPMU_PLUGIN_DIR); // sanitize for Win32 installs
+	$mu_plugin_dir = preg_replace('|/+|','/', $mu_plugin_dir); // remove any duplicate slash
+	$file = preg_replace('#^' . preg_quote($plugin_dir, '#') . '|^' . preg_quote($mu_plugin_dir, '#') . '/#','',$file); // get relative path from plugins dir
 	return $file;
 }