From http://www.telerik.com/products/aspnet-ajax/faq.asp Can I use RadControls for ASP.NET AJAX controls side-by-side with the RadControls for ASP.NET? Yes, you can. We have designed and tested the ASP.NET AJAX Prometheus
controls to work with RadControls for ASP.NET and have not encountered
any issues running them together in a wide variety of scenarios.