Kod again.

*test.php*

<html>

<head>

<script src="jquery.chained.js" type="text/javascript" charset="utf-8"></script>
<script src="jquery_states.js" type="text/javascript" charset="utf-8"></script>
<script src="jquery.js" type="text/javascript" charset="utf-8"></script>

</head>

<body>


<select id="mark">
  <option value="">--</option>
  <option value="bmw">BMW</option>
  <option value="audi">Audi</option>
</select>
<select id="series">
  <option value="">--</option>
  <option value="series-3" class="bmw">3 series</option>
  <option value="series-5" class="bmw">5 series</option>
  <option value="series-6" class="bmw">6 series</option>
  <option value="a3" class="audi">A3</option>
  <option value="a4" class="audi">A4</option>
  <option value="a5" class="audi">A5</option>
</select>


</body>


</html>

/* jquery.chained.js  */

/*
 * Chained - jQuery non AJAX(J) chained selects plugin
 *
 * Copyright (c) 2010-2011 Mika Tuupola
 *
 * Licensed under the MIT license:
 *   http://www.opensource.org/licenses/mit-license.php
 *
 */

(function($) {

    $.fn.chained = function(parent_selector, options) {

        return this.each(function() {

            /* Save this to self because this changes when scope changes. */
            var self   = this;
            var backup = $(self).clone();

            /* Handles maximum two parents now. */
            $(parent_selector).each(function() {

                $(this).bind("change", function() {
                    $(self).html(backup.html());

                    /* If multiple parents build classname like foo\bar. */
                    var selected = "";
                    $(parent_selector).each(function() {
                        selected += "\\" + $(":selected", this).val();
                    });
                    selected = selected.substr(1);

                    /* Also check for first parent without subclassing. */
                    /* TODO: This should be dynamic and check for each parent */
                    /*       without subclassing. */
                    var first = $(parent_selector).first();
                    var selected_first = $(":selected", first).val();

                    $("option", self).each(function() {
                        /* Remove unneeded items but save the default value. */
                        if (!$(this).hasClass(selected) &&
                            !$(this).hasClass(selected_first) && $(this).val() !== "") {
                                $(this).remove();
                        }
                    });

                    /* If we have only the default value disable select. */
                    if (1 == $("option", self).size() && $(self).val() === "") {
                        $(self).attr("disabled", "disabled");
                    } else {
                        $(self).removeAttr("disabled");
                    }
                    $(self).trigger("change");
                });

                /* Force IE to see something selected on first page load, */
                /* unless something is already selected */
                if ( !$("option:selected", this).length ) {
                    $("option", this).first().attr("selected", "selected");
                }

                /* Force updating the children. */
                $(this).trigger("change");

            });
        });
    };

    /* Alias for those who like to use more English like syntax. */
    $.fn.chainedTo = $.fn.chained;

})(jQuery);


/* jquery.js */

Fil klar


/* jquery_states.js */

$(function(){
     $("#series").chained("#mark"); /* or $("#series").chainedTo("#mark"); */
});