We investigated the ability of endogenous and recombinant interleukin-2 (IL-2)-stimulated NK cells from normal donors and ovarian cancer patients to mediate lysis of ovarian tumors, and found that peripheral blood (PB) mononuclear cells of normal donors or cancer patients did not display tumoricidal activity against ovarian cell line or fresh ovarian tumors. In addition, ovarian cancer patients displayed a defect in NK-cell activity against the highly NK-sensitive target, K-562. However, lytic activity against ovarian tumors (including cultured and primary tumors) was induced and that against K-562 was potentiated in PB of normal donors and PB and peritoneal cavity of ovarian cancer patients after enrichment of LGL on Percoll density gradient or after stimulation of effector cells with recombinant IL-2 in vitro. The IL-2-generated cytotoxic cells in PB were characterized as NK cells displaying CD16+, NKH1 (Leu-19)+, CD3- and CD5- phenotype, while those in the peritoneal cavity were predominantly CD16-, NKH1+, CD3- and CD5-. Studies on the mechanism of IL-2-dependent generation of cytotoxicity showed that such an effect was mediated via recruitment of tumor-binding cells as well as by an increase in the frequency of cytotoxic cells.