The characteristics and proportions of human lymphocyte populations capable of lysing IgG coated target cells were studied. Monolayer of target erythrocytes coated with rabbit IgG were employed to detect and isolate the lytic effector lymphocytes. Normal peripheral blood effector lymphocytes were shown to express IgG receptors, and a total of four cell types possesing different surface markers were identified. An average of 68% (range 51-80%) of the effector lymphocytes had T lymphocytes antigen and were capable of SRBC rosette formation, and 32% of the lymphocytes lacked T lymphocyte markers. Two populations of these T lymphocytes were identified: one possesses SRBC and IgG receptors, and the other possesses SRBC, IgG and complement receptors. The proportion of the former was estimated as 53.2% and the latter as 14.8% of the total effector cell population. Two other cell types were distinguished among the non-T lymphocytes. One type of lymphocytes bear IgG receptors (21.6%) and the other has both IgG and complement receptors (10.4%). The cytolytic capacity of the two T lymphocyte subpopulations is suggested to be an important reaction of the immune reactive T lymphocytes during the immune response.