In earlier experiments it has been shown that cellular cytotoxicity of patients with prostatic carcinoma (CaP) as measured by cytolysis of EB 33 target cells correlates with the extent of their tumor lesion: a high grade of cytotoxicity was observed only when CaP was confined to gland. In order to define a possible in vivo immunological host-tumor interaction, further characterization of the type of effector cell responsible for in vitro killing has been attempted. Effector cells could be either tumor-specific sensitized T cells, K cells, macrophages or NK cells. Antibody-dependent K-cell activity could most likely be excluded, since autologous and homologous serum did not affect the extent of EB 33 killing when added to the test medium. Further separation of effector cells by nylon wool columns resulted in increased cytolysis of EB 33 target cells. This can be due either to tumor-specific T-cell-mediated mechanisms or to an increased natural killer cell activity, as macrophages and B cells were significantly removed by this measure. Separation of T and NK cells is necessary to ascertain the effector cell responsible for the in vitro killing of tumor cells derived from human CaP.