bellow code is i used. It is working properly n fire fox and IE 8 . but IE 9 and IE 10 it is not working. I have used telerik version 2008.1.415.35
<telerik:RadComboBox ID="RadComboBox1" runat="server" Width="590px" Height="140px" AllowCustomText="false"
MarkFirstMatch="true" AutoCompleteSeparator=";"
EmptyMessage="Type an E-mail" DataSourceID="SqlDataSource1" DataTextField="DSG_NAME"
OnItemDataBound="RadComboBox1_ItemDataBound">
</telerik:RadComboBox>
<telerik:RadComboBox ID="RadComboBox1" runat="server" Width="590px" Height="140px" AllowCustomText="false"
MarkFirstMatch="true" AutoCompleteSeparator=";"
EmptyMessage="Type an E-mail" DataSourceID="SqlDataSource1" DataTextField="DSG_NAME"
OnItemDataBound="RadComboBox1_ItemDataBound">
</telerik:RadComboBox>