The selective cytotoxic activity of tumor necrosis factor (TNF) on many transformed human or murine cell lines is (almost) not species-specific. There are, however, a limited number of biological assay systems such as gene induction in murine thymoma cells and the murine thymocyte proliferation assay, in which recombinant murine (rm) TNF, but not recombinant human (rh) TNF is active. We have now investigated the possible species specificity of the lethality-inducing properties of TNF in the mouse. When administered alone, only rmTNF, but not rhTNF caused lethality. This difference was not due to a different endotoxin contamination or to a different pharmacokinetic behavior. When a sensitizing agent, galactosamine, was added, the species specificity was abolished. We have shown previously that similar conclusions could be drawn for at least some of the antitumor mechanisms of TNF. We conclude that to mimic the two major phenomena caused by endotoxin administration, viz. lethality and antitumor effect, two distinct signals are needed. In mice, rmTNF can provide both, while rhTNF needs another agent (a sensitizer) to cause systemic toxicity. The results are discussed with respect to recent findings showing that, in the mouse, rhTNF can only bind to the TNF-R55 receptor type, while rmTNF can bind both to the TNF-R55 and TNF-R75 receptor types.