or
<asp:CheckBox ID="Datenschutz" runat="server" /><asp:CustomValidator ClientValidationFunction="ValidateDatenschutz" runat="server" ID="DatenschutzValidator" CssClass="..." ErrorMessage="<br/>..." /><telerik:RadButton ID="Submit" runat="server" Text="Speichern" OnClick="OnSubmitClick" CausesValidation="true" />function ValidateDatenschutz(sender, eventArgs) { eventArgs.IsValid = document.all[datenschutzCheckboxId].checked;}