Studies were carried out to investigate the nature of the lymphocyte membrane receptor involved in the specific lysis of [51Cr]EL4 target cells by BALB/c cytotoxic peritoneal exudate lymphocytes (PEL) in vitro. In confirmation of earlier related studies, the cytolytic reaction is markedly inhibited by trypsin treatment (1 mg of trypsin/10(6) PEL/ml; 37 C for 30 min) of the PEL. Trypsinized PEL recover their cytolytic activity after 6 hr of incubation in trypsin-free medium at 37 C; recovery is suppressed at 4 C. Significantly, trypsinization of PEL inhibits their ability to adsorb to EL4 target monolayers, indicating that lymphocyte-target cell binding is a trypsinsensitive step of the cytolytic interaction. Specificity of binding for EL4 is restored upon recovery from trypsin treatment. In addition, trypsin treatment, under conditions sufficient to inhibit cytolysis, causes a drastic nonspecific deletion of iodinatable membrane species from the surface of PEL. Thus, a direct correlation between altered cytolytic function and membrane structure cannot be made. Interestingly, trypsinized PEL retain the capacity to kill EL4 to which they are agglutinated by concanavalin-A, suggesting that the lytic mechanism, as distinguished from the binding receptor, is still intact. Papain and pronase also have an inhibitory effect on cytolysis, while neuraminidase causes a significant augmentation of lymphocyte-mediated cytolysis.