codice:
<!doctype html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Esempio</title>
<script type="text/javascript">
function checkRadioboxes () {
	var bChecked = true;
	for (var nItem = 0; nItem < this.length && !this[nItem].checked; bChecked = this[nItem++].checked);
	if (!bChecked) { alert("Devi selezionare almeno un\'opzione."); }
	return bChecked;
}
</script>

</head>
<body>

<form  method="get" action="paginasuccessiva" onsubmit="return checkRadioboxes.call(this.mgruppo);">
<input type="radio" name="mgruppo" value="1" />bottone1	     
<input type="radio" name="mgruppo" value="2"/>bottone2
<input type="submit" name="Submit" value="submit" />
</form>

</body>
</html>