16,16-Dimethyl prostaglandin E2, a known cytoprotective agent, was examined for its ability to protect the liver against complement-mediated necrosis induced by an intravenous injection of a monoclonal antibody against a rat liver—specific antigen in rats. The hepatic injury induced by the antibody was characterized by (a) rapid development of numerous massive hemorrhagic foci of necrotic liver cells, (b) marked increases in serum liver enzyme activities and (c) pronounced reduction in the CH50 level, presumably as a result of complement consumption in the liver. Pretreatment with 16,16-dimethyl prostaglandin E2 at intraperitoneal doses of 20 and 100 μg/kg suppressed the hepatic injury, as evidenced by markedly mitigated liver-cell necrosis and much smaller increases in the serum-enzyme activities compared with the values in diseased control animals. The prostaglandin analogue failed to prevent serum complement consumption in response to the antibody injection or affect the CH50 level at the preinjury stage, indicating that neither complement inactivation nor interference with the antigen-antibody reaction was involved in the hepatic protection. The hepatoprotective doses of 16,16-dimethyl prostaglandin E2 produced a significant increase in liver cyclic AMP content in a dose-related manner. In addition, intravenous dibutyryl cyclic AMP at 3 and 10 mg/kg dose-dependently prevented histological and biochemical changes in the hepatic damage without altering the rate of reduction in serum complement activity. Like 16,16-dimethyl prostaglandin E2, dibutyryl cyclic AMP did not affect the preinjury CH50 level. These results suggest that increased hepatocellular cyclic AMP is responsible, at least in part, for the hepatic protection afforded by 16,16-dimethyl prostaglandin E2 against the membrane-damaging insult of activated complement, possibly through the stabilization of the plasma membrane.