Enable/disbable Multiple Select List

i have a select element:

<select name="test1[]" size="5" multiple id="test1[]" >
and I have two radio buttons to enable/disable:
HTML Code:
<INPUT TYPE=radio NAME="f1" VALUE="option 1" (this.value)"><INPUT TYPE=radio NAME="f1" VALUE="option 2" (this.value)">
Javascript is as follow:
Code:
function Disab (val) {
frm=document.send;
if(val=="option 1") {
frm.test1.disabled=true;

} else {
frm.test1.disabled=false;}
}

It appears when I name the select element with '[]' it doesn't work. Is there another way to gather data from element via php or is there another way to identifiy the element to disable.