How To Change Css Properties Of A Content Loaded Using Ajax?

Using a form or specifics links, I can update the content of the div "mydiv" using an Ajax request (mootools 1.2) (var myRequest = new Request({...).
The "imported" content looks like:

<div id="div1">...</div><div id="div2">...</div>...

Once my ajax call is done and completed (content of "mydiv" is replaced as expected) I would like to change some css properties of this "imported content (ie. add a top margin to div2).

I tried to add to

onComplete: function(response) {			$('div1').setStyle('margin-top', '50px');			alert('Request completed successfully.');		}

Nothing occurs but javascript error telling that element div1 is null.
Am I missing something here or what should I do in order to be able to manipulate the "imported" data?

Thanks in advance for your help.
BM
Edited June 20, 2012 by moderator (see edit history)