HTTP status code for normal request on AJAX pages


I have a set of php pages that I use specifically for AJAX requests. I was wonder what would be the best HTTP error code for when the page is accessed through a normal request.

I have been using 404 at the moment, but the page does exist.

I was leaning towards either 403 - Forbidden or 501 - Not Implemented.

406 Not Acceptable might be valid in this case.

The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.