A method was developed to label the outer surface of chick embryo fibroblasts with fluorescamine. Cells were labeled in less than a minute without disruption of the monolayer. Polyacrylamide gel electrophoresis resolved two distinct areas of fluorescence: a group of high molecular weight polypeptides and several rapidly migrating species. The latter were demonstrated to be phospholipids by thin layer chromatography. Fluorescamine did not label internal components of the cell, as evidenced by two intracellular proteins which were found to be nonfluorescent. Intact normal cells were labeled threefold more than transformed cells, indicating a possible loss of reactive sites at the surface.