Human T lymphocytes were separated into two subsets on the basis of relative affinity for sheep red blood cells (E), and these T cell fractions were examined for cytotoxic reactivity against antibody-sensitized Chang liver cells (ADCC). High affinity E-rosette-forming cells (E-RFC) (55 ± 6% of peripheral blood mononuclear cells), capable of rosette formation despite elevated temperatures of incubation (29°C) and a limited concentration of E, contained few antibody-dependent cytotoxic cells (K cells). In contrast, low affinity E-RFC (23 ± 7% of mononuclear cell suspensions) requiring cool temperatures of incubation (4°C) and an excess of E to form rosettes, were highly enriched for ADCC activity. The majority of K cells exhibited low affinity interactions with E. T cells in thymus, tonsil, and lymph node formed high affinity E-rosettes and exhibited little reactivity in ADCC. Only peripheral blood and spleen contained easily identified low affinity E-RFC and antibody-dependent cytotoxic cells. The proportion of low affinity E-RFC in the peripheral blood of normal subjects correlated closely with reactivity in ADCC, making it possible to predict cytotoxic potential from the E-rosette pattern. These data indicate that the human K cell may belong to a previously unappreciated but functionally important subset of thymic dependent mononuclear cells.