Flow cytometry was performed as described previously (Willett et al., 1993). Samples were stained with either vpgl5 (anti-feline CD9), vpg33 (anti-feline CD4) or FMC56 (anti-human CD9, Sera-Lab, Crawley Down, U.K.). Primary antibodies were detected with fluorescein isothiocyanate conjugated F(ab')2 fragment of sheep antimouse IgG (Sigma). Analysis was performed on an EPICS Elite flow cytometer (Coulter Electronics Ltd, Luton, U.K.)