This is the client side code I’m using

<script type="text/javascript">

    //global variables for the countries Combo

    var locationsCombo;

    function pageLoad() {

        // initialize the global variables

        // in this event all client objects

        // are already created and initialized

        locationsCombo = $find("<%= RadComboLocation.ClientID %>");

    }

    function LoadLocations(combo, eventArqs) {

        var item = eventArqs.get_item();

        locationsCombo.set_text("Loading...");

        // this will fire the ItemsRequested event of the

        // locations combobox passing the countryID as a parameter

        locationsCombo.requestItems(item.get_value(), false);

    }

    function ItemsLoaded(combo, eventArqs) {

        if (combo.get_items().get_count() > 0) {

            // pre-select the first item

            combo.set_text(combo.get_items().getItem(0).get_text());

            combo.get_items().getItem(0).highlight();

        }

        combo.showDropDown();

    }

</script>  

This is the code for both combos on the ascx pag

<telerik:RadComboBox ID="RadComboCountry" runat="server" AppendDataBoundItems="True"

                        BorderStyle="None" CssClass="DropdownControlGridFormEditor" EnableScreenBoundaryDetection="False"

                        ListTextField="Text" ListValueField="Code" Skin="Outlook" OnClientSelectedIndexChanging="LoadLocations"

                        OnItemsRequested="RadComboCountry_ItemsRequested">

                        <CollapseAnimation Duration="200" Type="OutQuint" />

                    </telerik:RadComboBox>

<telerik:RadComboBox ID="RadComboLocation" runat="server" BorderStyle="None" CssClass="DropdownControlGridFormEditor"

                        DropDownWidth="150px" EnableScreenBoundaryDetection="False" ListTextField="Text"

                        ListValueField="Code" Skin="Outlook" OnClientItemsRequested="ItemsLoaded" OnItemsRequested="RadComboLocation_ItemsRequested">

                        <CollapseAnimation Duration="200" Type="OutQuint" />

                    </telerik:RadComboBox>

And this is the code in the code behind page

protected void RadComboCountry_ItemsRequested(object o, RadComboBoxItemsRequestedEventArgs e)

        {

           try{

            SFSUserControl.myUtil.BindCombos(RadComboCountry, "Country", true);

           }

           catch (Exception ex)

           {

               throw new Exception("RadComboCountry_ItemsRequested Error on " + Request.ServerVariables["Script_Name"].ToString() + "" + ex.Message);

           }

        }

        protected void RadComboLocation_ItemsRequested(object o, RadComboBoxItemsRequestedEventArgs e)

        {

            try{

                if (SFSUserControl.myUtil.LocationItemExists("Location" + e.Text) == true)

                {

                    SFSUserControl.myUtil.BindCombos(RadComboLocation, "Location" + e.Text, true);

                }

                else

                {

                    RadComboLocation.SelectedValue = "None";

                    RadComboLocation.Enabled = false;

                }

             }

            catch (Exception ex)

            {

                throw new Exception("RadComboLocation_ItemsRequested Error on " + Request.ServerVariables["Script_Name"].ToString() + "" + ex.Message);

            }

        }