To determine functional relationships between the cytotoxic T lymphocyte (CTL) receptor for target binding and the lytic mechanism, we have studied the reaction between two immunized CTL populations (AαB and BαA), both at the population and the single-cell level. When studied at the population level, the reaction of AαB with BαA (bidirectional system) resulted in formation of AαB/BαA conjugates and bidirectional cytolysis. However, when the viability of cells in individual AαB/BαA conjugates was analyzed, unidirectional instead of bidirectional lysis occurred. These results indicate that under conditions that are conducive to lysis, binding of a potentially lytic cell to its target does not necessarily result in target lysis. Short heat treatment of CTL (44°C, 10 min) totally abolished their lytic activity, without affecting their capacity to bind specifically, thus dissociating the binding from the lytic activity of the CTL. The cytolytic activity is probably associated with, or triggered by the CTL-binding unit. The binding unit, on the other hand, appears to be a functional receptor of the CTL, which is involved in but not sufficient to bring about lysis.