Treatment with D-galactosamine increases sensitivity of lipopolysaccharide (LPS)-responder mice to the lethal effects of LPS, while nonresponder mice remain resistant (M.A. Freudenberg, D. Keppler, and C. Galanos, Infect. Immun. 51:891-895, 1986). In the present study it is shown that, in contrast to LPS, killed gram-negative bacteria (Salmonella abortus equi and S. typhimurium) were highly toxic for D-galactosamine-treated LPS-responder (C57BL/10 ScSN and C3H/HeN) and -nonresponder (C57BL/10 ScCR and C3H/HeJ) mice, although to a higher extent in the former strains. Also, killed gram-positive bacteria (Staphylococcus aureus, Propionibacterium acnes, and Mycobacterium phlei) exhibited toxicity for D-galactosamine-treated mice, LPS-responder and -nonresponder mice being equally susceptible. Evidently, bacterial components other than LPS may exhibit lethal effects in sensitized animals. In all cases, the lethality of LPS and of bacteria was inhibited by anti-tumor necrosis factor alpha (TNF-alpha) serum. While LPS induced TNF-alpha in vitro only in macrophages from LPS-responder mice, gram-negative and gram-positive bacteria induced TNF-alpha also in macrophages from LPS-nonresponder mice. The data show that TNF-alpha is a common endogenous mediator of the lethal activity of gram-negative and gram-positive bacteria.