A photoactivable glycolipid probe, 12-(4-azido-2-nitrophenoxy)stearoyl[1-14C]glucosamine, was used to label proteins and lipids of platelet membranes. The proteins were analyzed by two-dimensional high-resolution gelelectrophoresis. The labeling patterns showed that three membrane proteins were labeled which were not previously identified by ectolabeling (Sixma, J.J. and Schiphorst, M.E. (1980) Biochim. Biophys. Acta 603, 70-83). Analysis of the lipid fraction showed that phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine were labeled by the probe. The distinct labeling of phosphatidylserine strongly suggests that the probe redistributes between the two halves of the bilayer.