diff --git includes/bootstrap.inc includes/bootstrap.inc
index a710e0a..b64245f 100644
--- includes/bootstrap.inc
+++ includes/bootstrap.inc
@@ -1123,11 +1123,36 @@ function drupal_unpack($obj, $field = 'data') {
 /**
  * Encode special characters in a plain-text string for display as HTML.
  *
- * Uses drupal_validate_utf8 to prevent cross site scripting attacks on
+ * Also validates strings as UTF-8 to prevent cross site scripting attacks on
  * Internet Explorer 6.
+ *
+ * @param $text
+ *   The text to be checked or processed.
+ * @return
+ *   An HTML safe version of $text, or an empty string if $text is not
+ *   valid UTF-8.
+ * @see drupal_validate_utf8().
  */
 function check_plain($text) {
-  return drupal_validate_utf8($text) ? htmlspecialchars($text, ENT_QUOTES) : '';
+  // We do not want to use drupal_static() since PHP version will never change
+  // during a request.
+  static $php525;
+
+  if (!isset($php525)) {
+    $php525 = version_compare(PHP_VERSION, '5.2.5', '>=');
+  }
+  // We duplicate the preg_match() to validate strings as UTF-8 from
+  // drupal_validate_utf8() here. This avoids the overhead of an additional
+  // function call, since check_plain() may be called hundreds of times during
+  // a request. For PHP 5.2.5+, this check for valid UTF-8 should be handled
+  // internally by PHP in htmlspecialchars().
+  // @see http://www.php.net/releases/5_2_5.php
+  // @todo remove this when support for either IE6 or PHP < 5.2.5 is dropped.
+
+  if ($php525) {
+    return htmlspecialchars($text, ENT_QUOTES, 'UTF-8');
+  }
+  return (preg_match('/^./us', $text) == 1) ? htmlspecialchars($text, ENT_QUOTES, 'UTF-8') : '';
 }
 
 /**
diff --git modules/simpletest/tests/common.test modules/simpletest/tests/common.test
index 863d251..fc1c1a7 100644
--- modules/simpletest/tests/common.test
+++ modules/simpletest/tests/common.test
@@ -70,6 +70,42 @@ class CommonLUnitTest extends DrupalUnitTestCase {
   }
 }
 
+/**
+ * Tests for the check_plain() and filter_xss() functions.
+ */
+class CommonXssUnitTest extends DrupalUnitTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'String filtering tests',
+      'description' => 'Confirm that check_plain() and filter_xss() work correctly, including invalid multi-byte sequences.',
+      'group' => 'System',
+    );
+  }
+
+  /**
+   * Check that invalid multi-byte sequences are rejected.
+   */
+  function testInvalidMultiByte() {
+     $text = check_plain("Foo\xC0barbaz");
+     $this->assertEqual($text, '', 'check_plain() rejects invalid sequence "Foo\xC0barbaz"');
+     $text = check_plain("Fooÿñ");
+     $this->assertEqual($text, "Fooÿñ", 'check_plain() accepts valid sequence "Fooÿñ"');
+     $text = filter_xss("Foo\xC0barbaz");
+     $this->assertEqual($text, '', 'filter_xss() rejects invalid sequence "Foo\xC0barbaz"');
+     $text = filter_xss("Fooÿñ");
+     $this->assertEqual($text, "Fooÿñ", 'filter_xss() accepts valid sequence Fooÿñ');
+  }
+
+  /**
+   * Check that special characters are escaped.
+   */
+  function testEscaping() {
+     $text = check_plain("<script>");
+     $this->assertEqual($text, '&lt;script&gt;', 'check_plain() escapes &lt;script&gt;');
+  }
+}
+
 class CommonSizeTestCase extends DrupalUnitTestCase {
   protected $exact_test_cases;
   protected $rounded_test_cases;