Experimental hepatic fibrosis was produced in the guinea pig. We produced hepatic necrosis associated with inflammatory cell infiltration in guinea pigs immunized with acetaldehyde adducts and fed ethanol for 40 days. Extending the period of these treatments to 90 days resulted in producing hepatic fibrosis developing around individual hepatocytes in the terminal hepatic venule areas and portal areas, accompanied by an increase in hepatic hydroxyproline content. In contrast, no fibrosis was observed in the livers of the control groups that had been exposed to nothing, ethanol alone, or a combination of ethanol and immunization with unmodified human hemoglobin. Minimal fibrotic changes were observed in animals immunized with human hemoglobin acetaldehyde adducts but not fed ethanol. These results indicate that the formation of acetaldehyde adducts and the acquisition of immunity against them can produce hepatic fibrosis. Immune mechanisms against acetaldehyde adducts may, in part, be involved in the pathogenesis of hepatic fibrosis seen in alcoholics.