Inhibition of antibody-dependent cellular cytotoxicity (ADCC) by autologous lymphocytes was analyzed by using classical techniques for enzyme-substrate interactions. We determined empirically that the interaction of murine spleen cells with antibody-coated targets to produce lysis was analogous to the interactions that have been described for an enzyme with its substrate. Varying numbers of antibody-coated target cells (“substrate”) were mixed with a constant number of spleen cells (“enzyme”) and the number of target cells killed (“product”) was measured as a function of time. By analogy with Michaelis-Menten enzyme kinetics, two parameters of the reaction were determined: Vmax, the maximum velocity of lysis that is proportional to the number of killer cells present, and K½, an intrinsic property of the killer cells. These parameters were found to be independent variables. Addition of autologous lymph node cells produced a dose-dependent decrease in Vmax whereas K½ was not significantly changed. By analogy with enzyme kinetics, this inhibition is noncompetitive, suggesting that the autologous lymphocytes inactivate the killer cells rather than competing for the cell-cell binding sites.