C3, factor B, and alpha-1-antitrypsin were determined in newborn infants with septicaemia and sclerema, associated with suspected infections, ABO or Rh incompatibility, and hyperbilirubinaemia of unknown origin, during and after treatment with exchange transfusion. Activation products from C3 and factor B, the clearance of the transfused C3, and its synthesis by the recipient were determined also. Infected newborn infants had low levels of C3 and factor B, but a normal amount of alpha-1-antitrypsin. Exchange transfusion lowered the level of alpha-1-antitrypsin and briefly corrected the low level of C3 and factor B. Activation products were formed only exceptionally. As synthesis of C3 is very active, a defective activation of complement pathway linked to an abnormal distribution in extravascular pool is postulated.