We investigated the cytotoxic activity of recombinant human lymphotoxin (rHuLT) against Meth A sarcoma in vitro and in vivo. In direct cytotoxic assay, Meth A cells showed very low, if any, sensitivity to rHuLT. However, rHuLT was found to be strongly cytotoxic for subcutaneously implanted Meth A cells. In order to explain this discrepancy, we examined the indirect effects of rHuLT on Meth A cells particularly through macrophages. rHuLT showed macrophage chemotactic activity in vitro and also in vivo. Thus, injection of rHuLT into the mouse peritoneal cavity caused migration of macrophages to the peritoneal cavity. In addition, histologic analysis revealed that 6 h after intratumor injection of rHuLT, nonspecific esterase-positive cells started to migrate into Meth A tumor tissues, and after a further 24 h, a larger number of nonspecific esterase-positive cells had migrated into the Meth A tumor tissues. Moreover, peritoneal exudate cells induced by rHuLT showed considerably higher cytotoxic activity against Meth A cells than proteose peptone- or phosphate-buffered saline-induced peritoneal exudate cells. Furthermore, the cytotoxic activity of peritoneal exudate cells was found to reside in macrophages. Investigation of the mechanism of macrophage activation by rHuLT revealed that rHuLT can activate macrophages for cytotoxicity synergistically with mouse interferon-γ. These results strongly suggest that rHuLT, when injected into the tumor, can induce the migration of macrophages into the tumor and activate macrophages in situ to kill tumor cells.