Human peripheral blood lymphocytes were separated into subpopulations enriched or depleted with respect to B lymphocytes (Ig-bearing cells), T lymphocytes, (cell forming rosettes with sheep erythrocytes: E-RFC) and Fc receptor-bearing lymphocytes (EA-RFC). From the distributions and recoveries of the various cell types it could be concluded that there was very little overlap between Ig-bearing lymphocytes and EA-RFC. The latter cells partly belonged to "null" (non-T, non-B) cells; it was however demonstrated that 30 % of the EA-RFC were T cells (E-RFC). The lytic capacity in antibody-dependent lymphocytotoxicity (ADL) was shown to correspond with the proportions of EA-RFC in the various fractions. Non-T cells showed enhanced ADL activity when compared to the unseparated cells. Purified T cells populations also displayed ADL activity. Since the latter could not be due to contaminating non-T cells, this activity was ascribed to Fc receptor-bearing T lymphocytes.