The simultaneous injection of monoclonal antibody 9.2.27, directed against a chondroitin sulfate proteoglycan preferentially expressed on human melanoma cells, and 2 X 10(7) mononuclear splenocytes, eradicated established, progressively growing human melanoma tumors in nude mice. Neither splenocytes nor antibody alone achieved significant tumor regression. The cells responsible for tumor elimination are most likely natural killer (NK) cells: they are present in splenocytes of T cell-deficient nude mice, and cloned cells with NK activity are able to suppress tumor growth. Moreover, splenocytes treated with anti-asialo GM1 and complement or harvested from NK-deficient C57BL/6 beige mice did not cause tumor rejection. Furthermore, treatment of BALB/c nude mice just before injection with anti-asialo GM1 antiserum, which is known to eliminate NK activity in vivo, resulted in better tumor growth. In addition, evidence is presented that cells with NK activity are probably the effectors responsible for melanoma target cell lysis in vitro: Antibody-dependent and -independent cell-mediated lysis of M21 melanoma cells was suppressed when splenocytes were preincubated with complement and antibodies specific for cell surface antigens of NK cells, i.e., anti-asialo GM1, anti-Qa5, and anti-NK1.1. Moreover, splenocytes of C57BL/6 beige mice were not able to lyse M21 cells in vitro. These results strongly support the conclusion that cells with NK activity are indeed responsible for the antibody-dependent destruction of M21 melanoma cells in vivo and in vitro.