<asp:Button ID="Button1" runat="server" Text="Button" OnClick="Button1_Click" />

<asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>

<asp:CustomValidator ID="CustomValidator1" ControlToValidate="TextBox1"

    ClientValidationFunction="customValid"

    runat="server" ErrorMessage="CustomValidator"></asp:CustomValidator>

function customValid(sender, args) {

    $.ajax({

        type: 'POST',

        url: 'WebService1.asmx/IsOk',

        data: { str: args.Value },

        async: false,

        success: function(result) {

            args.IsValid = eval(result.text);

        }

    });

}

[WebMethod]

public bool IsOk(string str)

{

    return str == "ok";

}