KAI1 is a metastasis suppressor gene for human prostate cancer. To reveal the effect of KAI1 on the in vivo metastasis of tumors other than prostatic cancer, we transfected a human KAI1 cDNA into highly metastatic B16-BL6 murine melanoma cells and established stable transfectant clones with different expression levels of KAI1 message. The following results were obtained with the use of those transfectants. (1) Cell aggregation assay revealed a significantly enhanced Ca(2+)-independent aggregation of B16-BL6 cells by KAI1 cDNA transfection compared with mock transfectants (P < 0.01). (2) The in vivo phagokinetic activity and invasive ability of KAI1 transfectants were clearly decreased as compared with those of mock transfectants (P < 0.01). There was no significant effect of KAI1 expression on the in vitro or in vivo proliferation of B16-BL6 cells. (3) Lung colony formation of intravenously injected KAI1 transfectants in nude mice was significantly reduced as compared with mock transfectants or parental B16-BL6 cells (P < 0.01). These data suggest that KAI1 expression gives rise to the suppression of invasive and metastatic potentials of B16-BL6 cells.