[mod_python] sending post data using xmlhttprequest

I'm usung XmlHTTPRequest to send POST data.
I'm using util.FieldStorage to retrieve it.
The program returns an html page containing in part:
<h1>Cannot process request!</h1>
<p> The server does not support the action requested by the browser.

I've tried several varieties of post data, all with the same outcome.

What should XmlHTTPRequest post data look like?

I tested my server side code by sending post data from an html form, and 
that works OK.

-- 
Bob Gailer
510-978-4454