H-ras-transformed NIH3T3 cells that overexpressed a human melanoma-associated antigen ME491 (44-3H) were generated by transfection with the cloned ME491 antigen gene followed by a 'panning' selection, and effects of the antigen overexpression on H-ras-mediated malignant phenotypes were studied. Although in vitro growth properties of the 44-3H overexpresser cells, both anchorage-dependent and -independent, were practically the same as those of the 44-1C control cells, 44-3H cells exhibited less malignant phenotypes in athymic nude mice (in vivo), i.e. decreased tumourigenicity after subcutaneous inoculation and prolonged survival time after intraperitoneal inoculation, compared with 44-1C cells. These results suggested that overexpression of ME491 antigen partially suppressed malignant phenotypes of H-ras-transformed NIH3T3 cells in athymic nude mice through co-operating with a factor(s) or mechanism(s) that exist in vivo but not in vitro. Thus, ME491 antigen might act as a tumour suppressor under some circumstances.