We examined whether gammadelta T and alphabeta T cells accumulating in early B16 melanoma lesions regulate NK and NK T cells that attack tumor cells. Freshly isolated and cultured tumor-infiltrating lymphocyte (TIL) populations of NK and NK T cells lysed B16 and produced IFN-gamma, whereas gammadelta T and a large part of alphabeta T cell populations had no substantial cytotoxicity against B16 and secreted Th2 cytokines. Furthermore, the freshly isolated NK1 VSports手机版. 1+ TIL population exhibited a higher anti-B16 effect than did splenocytes. Gammadelta T and alphabeta T cell populations dramatically inhibited the cytotoxicity of NK and NK T cells in an MHC Kb-dependent manner. Culture supernatant from gammadelta T and alphabeta T cell populations inhibited the proliferation of NK and NK T cell populations but did not affect their cytotoxicity, suggesting that the released Th2 cytokines are merely partly involved in the down-modulation of NK-lineage cells. NK1. 1+ cells obtained from TIL of gammadelta T cell-depleted mice significantly lysed B16 cells compared with those from control mice. Finally, anti-Kb Fab mAb injected intralesionally at an early, but not at a late, stage of development of B16 melanoma inhibited tumor growth. These findings suggest that Th2-type gammadelta T and alphabeta T cells infiltrating in early B16 development inhibit the tumoricidal activity of NK-lineage cells using their class I molecules and partly their suppressive cytokines. .