Aldehyde-dehydrogenase activity of blood serum of practically healthy people, patients with chronic alcoholism (stage II) and in the experiment with intact animals (white rats) were studied in comparative aspect. Optimal conditions have been chosen to reveal aldehyde-dehydrogenase activity. It has been shown that aldehyde dehydrogenase of blood serum in healthy people manifests activity at very low (0.012 mM; aldehyde dehydrogenase 1), low (0.060 mM; aldehyde dehydrogenase 2) and high (1 mM; aldehyde dehydrogenase 3) concentrations of acetaldehyde. Activity of aldehyde dehydrogenase 1 is not revealed in patients with chronic alcoholism, while the values of activity of aldehyde dehydrogenase 2 and aldehyde dehydrogenase 3 are decreased. Aldehyde dehydrogenase 2 and aldehyde dehydrogenase 3 have been revealed in the rat blood serum. The apparent values Km and V have been calculated for aldehyde dehydrogenase 1 in people and aldehyde dehydrogenase 2 in rats when using acetaldehyde and NAD as the reaction substrates.