Prova questo:
File xml:
<?xml version="1.0" encoding="UTF-8"?>
<TESTI>
<ITEM id="1">Primo</ITEM>
<ITEM id="2">Secondo</ITEM>
<ITEM id="3">Terzo</ITEM>
<ITEM id="4">Quarto</ITEM>
<ITEM id="5">Quinto</ITEM>
</TESTI>
File Ajax:
Codice PHP:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
"http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<title>Untitled Document</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script language="JavaScript" type="text/JavaScript">
function getXmlHttpRequestObject() {
var receiveReq=false; //Clear our fetching variable
try {
receiveReq = new ActiveXObject('Msxml2.XMLHTTP'); //Try the first kind of active x object…
} catch (e) {
try {
receiveReq = new ActiveXObject('Microsoft.XMLHTTP'); //Try the second kind of active x object
} catch (e) {
receiveReq = false;
}
}
if (!receiveReq && typeof XMLHttpRequest!='undefined') {
receiveReq = new XMLHttpRequest(); //If we were able to get a working active x object, start an XMLHttpRequest
}
return receiveReq;
}
var receiveReq = getXmlHttpRequestObject();
function getNews( doc , id)
{
var itemContent = new Array();
//If our XmlHttpRequest object is not in the middle of a request, start the new asyncronous call.
if (receiveReq.readyState == 4 || receiveReq.readyState == 0)
{
//Setup the connection as a GET call to your page
//True explicity sets the request to asyncronous (default).
receiveReq.open('GET', doc , true);
//Set the function that will be called when the XmlHttpRequest objects state changes.
receiveReq.onreadystatechange = function()
{
if(receiveReq.readyState==4)
{
// if status == 200 display text file
if(receiveReq.status==200)
{
data = receiveReq.responseXML.getElementsByTagName('ITEM');
for(var i=0;i<data.length;i++)
{
itemContent[parseInt(data[i].getAttribute('id'))] = data[i].firstChild.nodeValue;
}
alert(itemContent[id]);
}
}
}
//Make the actual request.
receiveReq.send(null);
}
}
window.onload = function()
{
getNews("item.xml",1);
}
</script>
</head>
<body>
</body>
</html>

PS.
Non fare caso ai commenti just recycled 
Volendo a getNews puoi passare anche altri parametri.