The acute phase proteins alpha 1-acid glycoprotein (alpha 1-AGP) and alpha 1-antitrypsin (alpha 1-AT) were shown to inhibit, by a mechanism unidentified to date, the lethality induced by TNF both in normal mice and in mice sensitized with galactosamine. We found that both bovine alpha 1-AGP and human alpha 1-AT also inhibited specifically the induction of apoptosis of hepatocytes by TNF/ galactosamine in vivo. This inhibition is specific for TNF, since apoptosis induced by TNF and actinomycin D was also inhibited, while similar apoptosis of hepatocytes induced by anti-Fas remained unaffected. The observation that these acute phase proteins did not affect the induction by TNF of IL-6, nitric oxide, or serum amyloid P excludes a nonselective inhibition of the TNF-activated pathways. The protection conferred by alpha 1-AGP and alpha 1-AT is presumably indirect, since these proteins did not inhibit TNF/actinomycin D-induced apoptosis in the hepatoma cell lines HepG2 and BWTG3.