function getElementsByClass(searchClass,node,tag) { 	 //Dustin Diaz's getElementsByClass implementation
	var classElements = new Array();
	if ( node == null )
		node = document;
	if ( tag == null )
		tag = '*';
	var els = node.getElementsByTagName(tag);
	var elsLen = els.length;
	var pattern = new RegExp("(^|\\s)"+searchClass+"(\\s|$)");
	for (i = 0, j = 0; i < elsLen; i++) {
		if ( pattern.test(els[i].className) ) {
			classElements[j] = els[i];
			j++;
		} //if
	} //for
	return classElements;
}


if(document.getElementsByClassName) {
	//Firefox 3  C++ native implementation
	var classes_ = document.getElementsByClassName("someClass");
} else {
	//Dustin Diaz's getElementsByClass implementation (written above)
	var classes_ = getElementsByClass("someClass", document, "someTAG");
}

таким кодом пользуюсь в своей работе.