Interactions between the cellular and humoral compartments of the human immune system are well documented. Leukoregulin (LR) is a hormone with the ability to upregulate the natural killer (NK) phenomenon by increasing target cell sensitivity to NK lymphocyte cytotoxicity. In the present report, the interactions between LR and human cytotoxic T-lymphocytes (CTLs) are described. Two cultured T-cell populations are specifically cytotoxic for autologous human melanoma and fail to lyse allogeneic melanoma, K562, or autologous peripheral blood lymphocytes (PBLs) in 4 h chromium release assays. Pretreating allogeneic melanoma or K562 with LR resulted in 19-67% lysis at an effector:target (E:T) ratio of 5:1, while no more than 10% lysis was observed without LR. Autologous PBLs were also subject to lysis when pretreated with LR (50% at an E:T of 40:1 with LR, and 2% without LR). The observed effect was dose-related and was most effectively inhibited by autologous cold targets, but persisted in the presence of antibody to human lymphocyte antigen class I antigens (w6/32). Lysis of autologous melanoma was not increased by pretreatment with LR. LR appears to mediate lysis of melanoma, NK targets, and normal PBLs by tumor specific CTLs. The effect is dose-related and non-major histocompatibility complex-restricted. The data do not support a significant role for LR in the normal physiology of human CTLs, but the striking effects in vitro may prove to be experimentally or therapeutically useful.