Among methanol and its metabolites, formaldehyde was found to have the strongest inactivating effect on the activity of alpha1-antitrypsin preparation and inhibitor existing in blood serum. The influence of formaldehyde on the activity of serum alpha1-antitrypsin is lower in comparison with purified inhibitor. alpha1-Antitrypsin modified by formaldehyde inactivates the trypsin in its action on the BAPA to a smaller degree than on the hemoglobin. The effective formaldehyde concentration in the case of the BAPA is about 64 mM and in the case of the hemoglobin is about 256 mM. The significant inhibitory effect of methanol on alpha1-antitrypsin appears only at a high concentration of this compound. Formate does not decrease alpha1-antitrypsin activity. In people intoxicated with methanol, alpha1-antitrypsin activity decreases, whereas the content of this inhibitor does not change.