opacity not being inherited

I am attempted to make a semi-transparent div in IE/Firefox but IE is
only making the first image in the div opaque

for example if I have a div like:

<div class='trans'><img src="foo.jpg"><img
src="bar.jpg"><span>text</span></div>

only foo.jpg become opaque. any ideas why? It works as expected in
firefox

the css being applied is:

..trans{
width: 100%; opacity:.50;filter: alpha(opacity=50);
}