Two subpopulations of rabbit spleen and mesenteric lymph node T cells were identified by a monoclonal antibody, 8AC8. These subpopulations were separated on the flow cytometer and were analysed for their response to T cell mitogens and antigens, their responsiveness in mixed lymphocyte cultures, and their ability to function as cytotoxic effector cells. The 8AC8+ T cell subpopulation contained cells highly responsive to T cell mitogens, to antigen, and to allogeneic or autologous stimuli in a mixed lymphocyte reaction. In contrast, the 8AC8- T cell subpopulation was non-responsive to T cell mitogens, and responded, poorly to antigen, and to allogeneic and autologous stimuli in an MLR. Both the 8AC8+ and 8AC8- subpopulations exhibited xenogenic cytotoxic effector function. Thus, the 8AC8 MAb identified a subpopulation of mature differentiated rabbit T cells; these 8AC8+ cells share many of the characteristics of the human OKT4 helper/inducer T cell subpopulation.