The commercial source of fetal bovine serum used to supplement the growth medium of human skin fibroblasts alters the activity of the lysosomal enzyme dipeptidyl aminopeptidase-1 (DAP-1). Cells grown with one serum were found to have a threefold higher level of DAP-1 than those grown with serum from another source (P less than 0.001). The effect on DAP-1 activity was specific inasmuch as no differences were found in the activities of a variety of other lysosomal and nonlysosomal hydrolases: DAP-II, DAP-III, DAP-IV, beta-glucosidase, beta-glucuronidase, and N-acetyl-beta-galactosaminidase. The effect is reversible and is observed over a wide range of cell population doublings. Cell growth kinetics were not significantly different with the different sera.