The levels of alpha1-antitrypsin and alpha2-macroglobulin in the plasma of 129 newborns were determined. The infants were divided into 3 groups according to their perinatal history. In healthy newborns with an uneventful perinatal history the normal values for alpha1-antitrypsin were 1.97 +/- 0.44 g/l, and for alpha2-macroglobulin 3.11 +/- 0.69 g/l. No changes in these levels were found during the first week of life. The levels of alpha1-antitrypsin and alpha2-macroglobulin showed significant correlation to each other. In healthy newborns with different complications in the obstetric history the levels of alpha1-antitrypsin were not influenced, whereas alpha2-macroglobulin decreased slightly during the first week of life. The levels of alpha1-antitrypsin and of alpha2-macroglobulin showed no further correlation to each other. In sick term and preterm newborns (n = 18) alpha1-antitrypsin was increased in 5 of 7 babies suffering from bacterial infections and lowered in 4 of 9 cases with respiratory disturbances. Alpha2-macroglobulin was lowered in 15 babies. These results indicate different kinetics of the two antiproteases in vivo.