z-index issue in IE: low <input> displays above high <div> when inside relative positioned div

The code below is generated by an AJAX look up control.  When someone starts typing into the txtCategory <input> box, the asbMenu div pops up and is populated with options that match what they're typing.  If I try to add multiple lookup controls to a page, the <input> boxes display on top of the popup divs, despite having input box z-index = 1 and popup div z-index = 15000.  

This is only broken in IE.  In FF and Opera they are displaying as I would expect.

When I remove the outer div''s "position:relative", the z-index works (but isn't where I need it to be).

Thanks for any suggestions.

<div style="position:relative; top:10px; z-index:1">
<input name="txtCategory" id="txtCategory" type="text" maxlength="100" size="10" 
  class="global" autocomplete="off" style="width:210px; z-index:1" />
<br>
<div class='asbMenu' style='visibility:hidden' id='txtCategory_Menu'></div>
<input type="hidden" id="txtCategory_SelectedValue" name="txtCategory_SelectedValue" value="">
</div>
                    
.global {  font: 10pt Verdana }
      
.asbMenu {
		border: 1px solid lightblue;
		background: #ffffff;
		position: absolute;
		padding: 5;
		white-space: nowrap;
		z-index: 15000;
	}

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