Most studies of recognition events in cell-mediated cytolysis have analyzed the conditions that are required to establish mechanically stable binding between effector cells and target cells. This approach has shown that cytolytic T cells require a temperature above 20°C and free divalent cations to bind specifically to target cells. K cells on the other hand were shown to bind to IgG on target cells in the absence of divalent cations at 4°C. In this paper we shown that both K cell- and T cell-mediated cytolysis are inhibited by cytochalasin A, if this drug is added to cultures before the effector and target cells have interacted. However, the drug does not block the calcium-dependent lethal hit stage of cytolysis in either system. We then show that in both T cell- and K cell-mediated cytolysis, the cytochalasin A-inhibitable stage can proceed in the absence of divalent cations. We conclude from these data that: 1) both K cell- and T cell-mediated cytolysis have a divalent cation-independent stage, which is inhibitable by cytochalasin A; 2) firm mechanical binding of T effectors to target cells requires divalent cations. It is uncertain if the latter is necessary for cytolysis or whether K cells also have to go through a divalent cation phase before the lethal hit stage. The implications of these findings are discussed, especially as to the possibility that strong binding may not be necessary for T cell-mediated cytolysis.