.aspx
    <script type="text/javascript">
    function js()
    {
        var opts = document.all("rb");
        if(opts)
        {
            for(var i = 0; i < opts.length; i++)
            {
                if(opts[i].checked)
                {
                    alert(i+"\r"+opts[i].value);
                    break;
                }
            }
        }
    }
    </script>
    <form id="form1" runat="server">
    <asp:TextBox ID="tb1" runat="server"></asp:TextBox>
    <asp:RadioButtonList ID="rb" runat="server" RepeatDirection="Horizontal">
        <asp:ListItem Value="1a">a</asp:ListItem>
        <asp:ListItem Value="2b">b</asp:ListItem>
        <asp:ListItem Value="3c">c</asp:ListItem>
    </asp:RadioButtonList>
    </form>

.cs
        if (!IsPostBack)
        {
            tb1.Attributes.Add("onkeyup", "js()");
        }