Two human T lymphocyte subpopulations were purified on the basis of their different affinities to sheep erythrocytes and compared with unseparated and non-T lymphocytes as effectors in cell-mediated lympholysis (CML) after mixed lymphocyte culture (MCL), in antibody-dependent cellular cytotoxicity (ADCC) against the mouse mastocytoma line P 815, and in spontaneous cell-mediated cytotoxicity (SCMC) against the human myeloid leukaemia line K 562 and the Burkitt lymphoma line RAJI. The low-avidity T cells (T1) had developed into more potent effectors than the high-avidity (Te) when assessed in CML. In ADCC and SCMC both subsets exhibited low but consistent lysis with no demonstrable differences. Addition of human leucocyte interferon to the ADCC and SCMC cultures resulted in enhanced SCMC, most notable and to an equal extent with the T cell subsets. In contrast, ADCC of unseparated cells, non-T cells, and the two T subsets was found to be unchanged after interferon addition. These data are discussed in relation to findings with human T-cell subpopulations purified by other methods.