Validation of combo box

Required field validation using jquery is not working for combo box. I am using rference to validation engine.

<asp:ComboBox ID="cbBillTitle" runat="server"
                                        CaseSensitive="false"  cssClass="validate[required]"
                                          AutoPostBack="true">
                                           
                                           <asp:ListItem tEXT="SELECT" Value=" "></asp:ListItem>
                                              <asp:ListItem tEXT="mR" Value="MR"></asp:ListItem>
                                                 <asp:ListItem tEXT="mRS" Value="MRS"></asp:ListItem>
                                           
                                    </asp:ComboBox>
                                    
                                    
                                    <asp:Button ID="btnSubmit" runat="server" />