Abstract Aminopeptidase (AP) activity on rat natural killer (NK) cells was found to have the following characteristics: (1) the activity was surface associated and not secreted, as determined by extracellular location of product and by the cessation of hydrolysis of substrate upon removal of the cells from the medium. (2) The activity was linear with respect to time and cell number. (3) The enzymatic activity on splenocytes and on the NK leukemia cell line CRNK-16, but not on IL-2 activated NK (A-NK) cells, was sensitive to trypsin treatment. (4) The AP activity on intact cells had a broad pH dependency with optimal activity at slightly alkaline pH but lower activity at acidic pH. (5) There was a preference for neutral substrates and essentially no activity towards acidic substrates. (6) Enzymatic activity was inhibited in the presence of the AP inhibitors bestatin and amastatin, and in the presence of the chelator, 1,10 phenanthroline, indicating the involvement of a metalloprotease. (7) Culture of A-NK cells with bestatin resulted in a decrease in cytotoxicity against YAC-1 and P815 targets. Amastatin treatment caused only a slight decrease in cytotoxicity against YAC-1 targets, but a significant decrease in cytotoxicity against P815 targets. (8) Treatment of A-NK cultures with specific inhibitors of APases caused an increase in expression of CD2 (an increase from 20–80% with bestatin and an increase from 25–35% in the presence of amastatin). These results provide the first evidence for the existence of APases on the surface of NK cells and suggest a role for these enzymes in the regulation of cytotoxic activity and of CD2 surface expression.