How can I get instance variable return from ajax on ruby..

in AJAX, there is responseText, responseBody, responseXML,
responseStream,
so I don't know what I can use to get instance variable return from
ajax on ruby..
who can tell me, how can I do. Thanks!!

Thanks Daniel.
in my case, I just want to get @searchresult.

In controller:
search_controller
..
def index
..some codes.
end

def search
@searchresult=Search.find(:all)
some other codes.
end

in views:
search.rthml
<div id="a">
  <select id="a1"><option value="aa">aa
<option value="bb">bb
<option value="cc ">cc
</select>
<div id="result">
here just display @searchresult <====juse display @searchresult
(1)
</div>
</div>

when I used AJAX, that means I changed selected, I got all the changed
content, include index and search, displayed (1)
so I don't know what I can use to get instance variable return from
ajax. and I don't want to make XML file for this case. Just only want
to get @searchresult
to change view...