Padraig We have used Cy-labeled ligands extensively. They are extremely bright but do bleach rather quickly in aqueous media. Apparently, if you mount in something like methyl salicylate, there's less of a problem. The fluorescence signal may be bright enough to get away with using the 568 nm line to excite (about 11% of max absorbtion), in which case bleaching is reduced substantially. Jackson immunoreasearch sells many Cy5-labeled reagents, and you can label anything they don't sell using reactive dye kits available from Amersham/BDS. Feel free contact me for more information. Peter Sargent UCSF [log in to unmask]