function LoadGrainClasses() {
    $("#LotBase_DicGrainClassId").empty();
    var grain_type_id = $("#LotBase_DicGrainTypeId option:selected").val();
    if (grain_type_id != 0) {
        $("#LotBase_DicGrainClassId").attr('disabled', false);
        $.getJSON("/home/GetClassList", { grainTypeId: grain_type_id }, FillClassesBox());
    }
    else
        $("#LotBase_DicGrainClassId").attr('disabled', true);
}

function FillClassesBox(classesList) {
    //здесь classesList - undefined
    $.each(classesList, addClass())
}

function addClass(i, classname) {
    $("#LotBase_DicGrainClassId").append(classname.GrainClassName)
}

        public JsonResult GetClassList(int grainTypeId)
        {
            var a = _repository.GetDicGrainClasses().Where(c => c.DicGrainTypeId == grainTypeId).ToList();            
            //a - Здесь точно есть элементы
            return Json(a);
        }

JsonRequestBehavior.AllowGet