Aldehyde dehydrogenase (ALDH) enzymes from human liver homogenates were recognized in immunoblotting experiments and precipitated in Ouchterlony double diffusion gels by antibodies raised to the horse liver mitochondrial and cytosolic ALDH isozymes. The antibody raised to the cytosolic horse liver ALDH (alpha HC) has been shown to be specific for cytosolic ALDH isozymes, while the antibody raised to the horse liver mitochondrial ALDH (alpha HM) precipitated both mitochondrial and cytosolic ALDH isozymes. It was possible to selectively remove the cytosolic ALDH from a homogenate of a liver sample from a Caucasian by preincubation with alpha HC; the remaining mitochondrial enzyme was then precipitated by alpha HM in double diffusion gels. The experiments were repeated with a liver sample from an Oriental, presumed to have been alcohol sensitive since no active mitochondrial ALDH was found. The precipitation of a relatively inactive mitochondrial enzyme by alpha HM from a cytosolic ALDH-free sample confirmed previous reports of the existence of a mitochondrial ALDH in tissue from an alcohol-sensitive Oriental. The results of immunoblotting experiments confirm the co-migration, in electrophoresis, of the cytosolic and mitochondrial ALDHs from the liver of an alcohol-sensitive Oriental. The results reported here, together with previous observations, indicate that the antibodies raised to horse liver ALDH isozymes can be used to determine the subcellular location of ALDH isozymes in various human tissues, including frozen tissue samples which are not amenable to subcellular fractionation.