Alpha Transparency Bitmaps

I haven’t worked with AS3 for a few months, and even when I did I never took the time to learn it to extensively :|…

I’m creating an isometric map editor, the bitmap that I’m using to take the tiles from has a transparent background, but when I draw the tiles to the main bitmap it displays as white, so it messes up the overlapping (since they’re isometric)… How can I make the background of it transparent?

This is my function to grab the tile from the tile sheet.

private function getImageFromSheet(s:Number):Bitmap
			var sheetCols:int = tSheet.width / tWidth;
			var row:int = Math.floor(s / sheetCols);
			var col:int = s - (row * sheetCols);
			var rect:Rectangle = new Rectangle(col * tWidth, row * tHeightReal, tWidth, tHeightReal);
			var pt:Point = new Point(0, 0);
			var bmp:Bitmap =  new Bitmap(new BitmapData(tWidth, tHeightReal, true, 0));
			bmp.bitmapData.copyPixels (tSheet, rect, pt, null, null, true);
			return bmp;

Thanks for any help.