My js works in Mozilla but it doesn't in IE

The following script works in FF but not in IE:

This is the implementation of the functions that I use in a php file:

<script language="javascript" type="text/javascript">
function toonDeelrefGroep(visible)
{
if(visible)
{
deelref.style.display = (navigator.appName.indexOf("Microsoft")!=-1)?"block":'table-row';
deelrefdrop.style.display =(navigator.appName.indexOf("Microsoft")!=-1)?"block":'table-row';
deelrefspacer.style.display =(navigator.appName.indexOf("Microsoft")!=-1)?"block":'table-row';
}
else
{
deelref.style.display = 'none';
deelrefdrop.style.display = 'none';
deelrefspacer.style.display = 'none';
}
}

function checkDeelrefGroep()
{
if( document.wijzigReferentie.referentiegroep_id[0].selected == true )
toonDeelrefGroep(true);
else
toonDeelrefGroep(false);
}

</script>

The function call is like this:

if(($referentiegroep_id == $referentiegroep_id_option) && ($referentiegroep_id_option != 1 ))
{
$content .= "<option value='$referentiegroep_id_option' onclick=\"toonDeelrefGroep(false)\" selected>$titel_option";
}
elseif(($referentiegroep_id == $referentiegroep_id_option) && ($referentiegroep_id_option == 1 ))
{
$content .= "<option value='$referentiegroep_id_option' onclick=\"toonDeelrefGroep(true)\" selected>$titel_option";
}
else
{
$content .= "<option value='$referentiegroep_id_option' onclick=\"toonDeelrefGroep(false)\">$titel_option";
}

I dunno what is going on, all I want is a way to make real time descisions based on users selection. It al comes down to showing or not showing a option list based wat the user is selecting from anatothe roption list.

Does anyone has an idea of what it mightbe that i doesn't work in IE??

Thank u in advance,

KennyPHP