Aldehyde dehydrogenase isozymes (ALDH1 or E1 and ALDH2 or E2 according to the classification by Greenfield NJ, Pietruszko R, Biochim Biophys Acta 483:35, 1977) were purified from the human liver to homogeneity by the use of ion exchange chromatography on CM-Sephadex, DEAE-Sephadex, and QAE-Sephadex and affinity chromatography on 5'-AMP Sepharose 4B, and preparative isoelectric focusing agarose electrophoresis. These were injected in rabbits to elicit antibodies against ALDH1 and ALDH2, respectively and their specificities were tested by double immunodiffusion. By gel filtration, the antibodies were separated into F'ab fragments, conjugated with horseradish peroxidase, and served to detect ALDH1 and ALDH2 proteins in normal liver tissue. Immunoelectron microscopy by the preembedding method revealed that the electron-dense materials reacted with anti-ALDH1 antibody were located in the cytoplasm of hepatocytes, mainly around the nuclear membrane, mitochondria, and endoplasmic reticulum; in the mitochondria, however, no staining was demonstrated. In contrast, the reaction with anti-ALDH2 antibody was observed only in the mitochondria. Immunostaining, performed by the enzyme-labeled antibody method demonstrated that in the hepatic lobule, there were no differences of the intensity of ALDH1 antigenicity and that of ALDH2 antigenicity between periportal (zone 1) and pericentral (zone 3) hepatocytes. These results suggest that in the human liver ALDH1 and ALDH2 are distributed equally in the periportal and centrilobular regions.