Liver proteins form adducts with acetaldehyde and are modified by products of lipid peroxidation in alcohol-fed animals. It has been hypothesized that the formation of these modified liver proteins may contribute to liver injury in alcoholic liver disease. The present work was performed to determine the extent of protein modification in rats with experimental alcoholic liver disease. Rats were fed ethanol intragastrically with medium chain triglycerides (MCTs), palm oil, corn oil, or fish oil. The group fed MCTs and ethanol showed no liver injury, rats fed palm oil and ethanol showed only fatty liver, rats fed corn oil and ethanol showed fatty liver with moderate necrosis and inflammation, and rats fed fish oil and ethanol showed fatty liver with severe necrosis and inflammation. Antibodies were raised by using keyhole limpet hemocyanin modified in vitro by 4-hydroxynonenal (4-HNE) or acetaldehyde as immunogens. When liver extracts were examined by Western blot analysis, the intensities of the acetaldehyde-modified protein band (37 kd) in the alcohol-fed animals were significantly different among the ethanol-treated groups and correlated with plasma acetaldehyde concentrations. It was strongest in rats fed fish oil and ethanol, followed by rats fed palm oil and ethanol and rats fed corn oil and ethanol, whereas rats fed MCTs and ethanol showed the weakest intensity. The 37-kd protein-adetaldehyde adduct was located mainly in the pericentral region of the liver. No acetaldehyde adduct was detected in the control rats that were pair-fed with isocaloric amounts of dextrose. Western blot analysis using the anti-4-HNE antibody showed four distinctive bands (48, 45, 40, and 38 kd) in the liver extracts of alcohol-fed rats. Control animals showed only a weak 38-kd band. Although the intensities of the 48-, 40-, and 38-kd bands were similar among the different ethanol-treated groups, the intensity of the 45-kd band decreased from MCTs and ethanol > palm oil and ethanol > or = corn oil and ethanol > fish oil and ethanol. The data indicate that the degree of liver protein modification by acetaldehyde correlates well with the severity of liver injury in ethanol-fed rats, whereas modification by the lipid peroxidation product 4-HNE shows no correlation with the severity of liver injury.