The relationship between expression of Ia antigens or Fc gamma receptors (Fc gamma R) and different cytolytic activities of mixed leukocyte culture (MLC)-activated T-cell populations was studied. Lymphocytes mediating specific lysis of target cells bearing the stimulating alloantigens (CTL) could clearly be distinguished from cells mediating antibody-dependent cellular cytotoxicity (ADCC) in that CTL were Fc gamma R- while cells mediating ADCC were Fc gamma R+. In contrast, MLC-generated natural killer (NK) cells were distributed in both the Fc gamma R- and Fc gamma R+ cell fractions. The presence or absence of Ia antigens did not correlate with any of the cytolytic activities studied. In addition, Fc gamma R- and not Fc gamma R+ cells responded in the secondary MLC. Thus, although Fc gamma R+ cells are generated in large numbers in MLC, they do not appear to play a direct role in specific cytotoxicity nor do they proliferate in response to secondary stimulation. However, their ability, following MLC generation, to mediate NK activity and ADCC, both of which may contribute to in vitro and in vivo lysis of allogeneic cells, might explain their appearance following allogenic stimulation.