We produced hepatitis in guinea pigs by immunization with acetaldehyde adducts and ethanol treatment. Human hemoglobin-acetaldehyde adducts were prepared without any reducing agents and affinity purified with polyclonal antibodies against acetaldehyde adducts. Female guinea pigs were immunized with the adducts and were simultaneously given ethanol for 40 days. These treatments induced hepatic necrosis with infiltration of mononuclear cells in the hepatic lobules. The formation of the lymphoid follicle was also observed in severe cases. These changes were accompanied by the elevation of serum AST and lactic dehydrogenase activities and titers of circulating antibodies against acetaldehyde adducts. By contrast, the combination of ethanol and immunization with unmodified hemoglobin produced only fatty change of the liver, and animals immunized with the adducts alone had minimal inflammatory changes of the liver. Peripheral blood lymphocytes obtained from the animals with hepatitis were shown to be stimulated by acetaldehyde adducts to a significantly greater degree than those from control animals who received nothing, ethanol alone or ethanol and unmodified hemoglobin. These results suggest that the immune response to acetaldehyde adducts may be involved, at least partly, in the pathogenesis of inflammation observed in alcoholic liver disease.