function ajaxObject(){
	var xmlHttp=null;
	try
 	{
		xmlHttp=new XmlHttpRequest();
	}
			catch(e)
		{
			xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
			catch (e)
			{
				try
				{
					xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
					catch(e)
					{
						alert('הדפדפן לא תומך בAJAX');
						return false;
					}
				}
				}
		}
	return xmlHttp;
	}
function checkit()
{
	var xhr = ajaxObject();
	c = document.getElementById('c').selectedIndex;
	url = "ajax.php?id="+c;
	xhr.onreadystatechange=function()
	{
		if (xhr.readyState == 4 && xhr.status == 200)
		{
			document.getElementById('div').innerHTML = xhr.responseText;
		}
	}
	xhr.open('GET', url, true);
	xhr.send(null);
}
HTML CODE:
<select id="c" onChange="checkit();">
<option...>
<option..>
</select>