imageList1.TransparentColor = SystemColors.
this.gridDataBoundGrid1.SupportsTransparentBackColor = true;to get things to work. Can you send a little sample project showing what you are trying to do with your imagelist and the grid to email@example.com and mention this forum thread in the subject line? Them maybe we can spot teh problem you are having.
bitmap1.MakeTransparent(Color.White);on the bitmap if you want the bitmaps transparent color to be white.