The existence of proteolytic enzymes bound to surface of migrating cells has often been surmised. That such enzymes are present on mononuclear leukocytes was suggested by studies showing that serum amyloid A (SAA), the presumed precursor of amyloid protein A, is degraded in the presence of monocytes without endocytosis and with only negligible activity in the cells' supernates. Experiments using immunofluorescence were designed to support this view. It was shown that SAA binds to the cells' surface at low temperatures, whereas binding at 37°C could only be demonstrated when the cells were pretreated with the serine protease inhibitor, diisopropyl-fluorophosphate (DFP) or the elastase inhibitor Ac-Ala-Ala-Pro-Val-CH2Cl. Exposure of the cells to these inhibitors before incubation with SAA at 0°C permitted detection of the protein for an indefinite period of time. At 37°C the DFP treated cells polarized and eventually lost the surface-bound protein. No interiorized SAA could be demonstrated. Radioautography of cells that had been treated with 3H-DFP revealed grains on the plasma membrane and in the cytoplasm of sectioned monocytes, whereas only 8-15% of lymphocytes were labeled. In lymphocytes radioactivity was restricted to the surface membrane. Additional experiments showed that α-naphthyl acetate esterase activity is also located on the surface of monocytes and a subpopulation of lymphocytes. These observations have led to the conclusion that some functions of mononuclear leukocytes may be mediated by enzymes associated with the external surface of these cells.