The relationship between antigen recognition and lytic expression by killer T cells was studied by co-culturing two effector cell populations. When antigen recognition was bidirectional (e.g., b anti-d cells cultured with d anti-b) there was a loss of lytic activity in both populations. In contrast, when antigen recognition was unidirectional (e.g., a anti-d co-cultured with d anti-b) then the loss of lytic activity only occurred in that direction; i.e., there was a marked decline in the d anti-b activity but no change in the a anti-d population. These studies suggest: i) that mere proximity to a killer cell does not lead to target cell death; ii) that accommodation of the T cell's antigen receptor is necessary for the cell to express its lytic potential; and iii) there is direct linkage between the T cell's antigen receptor site and its killing mechanism.