SummaryWe have examined the antitumor and antimetastatic effects of native- type, glycosylated recombinant lymphotoxin (LT) on human and murine tumors transplanted in mice. The results reported here are as follows: (a) The in vivo antitumor spectrum of LT is not coincident with the in vitro study, and it has a wide antitumor spectrum and substantially inhibits the growth of human solid tumors, (b) When both syngeneic and nude mice are transplanted with Meth A tumor, the significant growth-inhibitory effect of LT is obtained in syngeneic mice, but the effect is quite small in nude mice regardless of the routes; LT attains the same degree of effectiveness as that in syngeneic mice, but at an 8 to 16 times higher dose. Furthermore, the pretreatment with antiasialo- GMl antibody inhibits the antitumor effects of LT in syngeneic mice, (c) In the pulmonary metastasis model induced by i.v. injection of Meth A cells, a high preventive effect of LT is obtained by systemic administration in syngeneic mice, but not in nude mice. In addition, the pretreatment with antiasialo- GMl antibody completely prevents the antimetastatic effect of LT, but also blocks that effect of control mice without LT treatment. In conclusion, LT appears to be a potent cytokine against tumor growth and metastasis in vivo. The differences between nude and syngeneic mice suggest the involvement of host immunity in the expression of LT function.