In a 51Chromium release assay performed without immune sera, purified lymphocytes from randomly selected healthy donors killed significantly more allogeneic virus-infected than noninfected fibroblasts. An incubation period of 18 to 20 hr was optimal for determining cytotoxicity, but increased cytotoxic activity against infected cells was observed as early as 6 hr from the start of the test. No correlation could be found between donors' antibody titers to any virus tested and the cytotoxic efficiency of their effector cells: virus-infected cells were lysed by both seropositive and seronegative individuals. With several different cell fractionation techniques, the effector cell in this system could not be distinguished from the human natural killer cell, which is a lymphocyte with receptors for the Fc fragment of IgG but with no surface immunoglobulin. When lymphocytes were separated on the basis of ability to form rosettes with neuraminidase or AET (2-aminoethylisothiouronium bromide hydrobromide)-treated sheep erythrocytes, the majority of cytotoxic activity was consistently recovered in the nonrosetting fraction. A portion of it, however, was always present in the rosetting fraction and was, again, mediated by lymphocytes carrying receptors for the Fc fragment of IgG.