TypeError: jQuery.toJSON is not a function

I'm trying to use jQuery and Ajax and I use this method. but I get the error $.toJSON is not a function in my firebug . where is the problem? I use jquery 1.3.2 . thanks

  $(document).ready(function () {
        $("#S1").click(function 
            () {
            $("#t1").slideToggle("fast");
            $("#S1").css("background-color", "yellow");
            var ID = $("#HiddenField2").attr("Value");
            var params = new Object();
            params.Key = ID;
            $.ajax({
                type: "POST",
                url: "viewMessages.aspx/readen",
                data: $.toJSON(params),
                contentType: "application/json",
                dataType: "json",
                success: function () {

                }
            });
        });
    });

There is no $.toJSON() function: http://api.jquery.com/jQuery.toJSON. Perhaps you want to use JSON.stringify() instead.
This error occurs because of jquery-json library missing. follow the url https://code.google.com/p/jquery-json to download.