problem in javascript

Hi I am posting multiple values from selection box(multi selection combo-box) to php and getting javascript error.
Detail:-

<select name="combo1[]" multiple>
....
Now there is Java-Script error when I call:-
document.frm1.combo1.selectedIndex='2';
Error:-
document.frm1.combo1 is null or not an object

I also tried as but get same error:-
document.frm1.combo1[].selectedIndex='2';