The cellular cytotoxicity mediated by mouse spleen cells in the presence of mitogenic or non-mitogenic lectins was established under serum-free conditions and characterized. Compared with the lectin-induced cellular cytotoxicity (LICC) in the guinea-pig, the activity of lymphotoxin (LT) released into the murine LICC assay cultures was very low. However, a positive correlation was found between the strength of LICC and the LT activity released into the supernatants. Moreover, the addition of puromycin, a potent enhancing reagent of guinea-pig LT activity, markedly promoted the LICC when added 4 h after the initiation of the LICC culture. These data, taken together, suggest that LT acts as an effector molecule in the murine LICC systems as well as in the guinea-pig LICC systems. Properties of the effector cell populations mediating LICC were investigated by depletion of plastic-adherent or nylon-wool adherent cells, by treatment of spleen cells with anti-T-cell sera and complement, and by use of nude mouse spleen cells. The results obtained suggest that both concanavalin A and phytohaemagglutinin-P can induce nylon-wool non-adherent T-cell mediated LICC, phytohaemagglutinin-W was found to be capable of inducing both the nylon-wool non-adherent T-cell mediated LICC and the nylon-wool adherent non-T-cell mediated LICC, the major effector of Bauhinia purpurea agglutinin-LICC was found to be the nylon-adherent non-T-cell population.