Radio button unchecked

[biblio]

Te paso este código para que lo veas.
<pre> &lt;html&gt;
&lt;head&gt;
&lt;title&gt; Uno solo &lt;/title&gt;
&lt;script language=&quot;JavaScript&quot;&gt;
&lt;!--
function checkForm(frm){
var x = 0;
for (i=0;i&lt;document.forms[0].length;i++){
if ((document.forms[0].elements[I].type) == &quot;radio&quot;){
x = x + (document.forms[0].elements.checked)?1:0;
}
}
if (x &lt; 1){
alert(&quot;No marcó ninguno&quot;);
return false;
}
else
document.forms[0].submit();
}
//--&gt;
&lt;/script&gt;
&lt;/head&gt;
&lt;body bgcolor=&quot;#FFFFFF&quot;&gt;
&lt;form method=&quot;post&quot; action=&quot;&quot; onsubmit=&quot;return checkForm(this);&quot;&gt;
&lt;p&gt;Marcar:&lt;/p&gt;
&lt;p&gt;
&lt;input type=&quot;radio&quot; name=&quot;radiobutton&quot; value=&quot;op3&quot;&gt;
&lt;input type=&quot;radio&quot; name=&quot;radiobutton&quot; value=&quot;op2&quot;&gt;
&lt;br&gt;
&lt;input type=&quot;submit&quot;&gt;
&lt;/p&gt;
&lt;/form&gt;
&lt;/body&gt;
&lt;/html&gt;
</pre>
Esta para dos, funciona con cuanto radio lo pongas.
Saludos.