Human liver alcohol dehydrogenase (ADH) exists in multiple molecular forms which arise from the association of eight different types of subunits, alpha, beta 1, beta 2, beta 3, gamma 1, gamma 2, pi, and chi, into active dimeric molecules. A genetic model accounts for this multiplicity as products of five gene loci, ADH1 through ADH5. Polymorphism occurs at two loci, ADH2 and ADH3, which encode the beta and gamma subunits. All of the known homodimeric and heterodimeric isoenzymes have been isolated and purified to homogeneity. Analysis of the steady-state kinetic properties and substrate and inhibitor specificities has shown substantial differences in the catalytic properties of the isoenzymes. For example, the Km values for NAD+ and ethanol vary as much as 1,000-fold among the isoenzymes. Some of the differences in catalytic properties can be related to specific amino acid substitutions in the ADH isoenzymes.