The study included 50 newborns with (RDS) and 41 controls in relation with the plasma concentration of alpha-antitrypsin (a-l-AT) and of proteins. The blood samples were taken at 24, 72 and 120 postnatal hours. At 24 hours, in infants with IRDS 187.5 +/- 9 mg/dl of a-l-AT were shown and 4.15 +/- 0.09 g. % of total proteins, which compared with concentrations of a-l-AT and total proteins in controls of the same age, turned out significantly lower. The a-l-AT and total proteins suffered increment in their concentrations at 72 hours of life in relation to control cases in whom there was no modifications in the course of postnatal age. Among other factors, a drop in total protein was considered related to the drop in globulin fraction. The usefulness of quantification of a-l-AT for the diagnosis of IRDS, is pointed out at the end.