Murine spleen cells developed into nonspecific cytotoxic cells within 72 hr of culture in the presence of highly purified sources of human interleukin 2. In whole spleen cell cultures, human interleukin 2 generated effector cells which were Thy 1.2+, Lyt 2.2+, resistant to gamma irradiation (1000 R), and capable of lysing both H-2 compatible and incompatible targets. The effector cells generated in this manner were not restricted to classical natural killer cell-sensitive targets. If thymus-derived cells (T cells) were depleted from the spleen cell population before culture with human interleukin 2, the effector cells generated were enriched in effectors capable of lysing natural killer cell-sensitive targets. Interferon was not produced in interleukin 2-stimulated spleen cell cultures. In addition, heterologous antibody to murine gamma-interferon did not abrogate the generation of cytotoxic cells by human interleukin 2. These and additional data suggest that human interleukin 2 is capable of stimulating gamma-irradiation-sensitive Thy 1.2+ cell(s) capable of lysing a variety of target cells regardless of inherent sensitivities to classical natural killer cells. Thy 1.2- cells were also stimulated by human interleukin 2 and lysed only natural killer cell-sensitive targets. Human interleukin 2 caused some Thy 1.2- cells to become susceptible to lysis by anti-Thy 1.2 serum and complement.