Concentrations of a peroxidation product (malondialdehyde), fluorescent chromophores, lipofuscin-like fluorescent products, superoxide dismutase, catalase, glutathione peroxidase, and vitamin E in the maternal blood and the cord blood were determined and the results obtained were related to the estimation of lipid peroxidation and protective mechanism against uncontrolled oxidative processes in late pregnancy. Serum levels of fluorescent products were higher in the maternal blood than in the cord blood, indicating less frequent lipid peroxidation in the fetus than in the mother. In support of this assumption, the three protective enzymes and vitamin E were present in relatively lower concentrations in the cord blood. Sudden exposure of the newborn infant to a normobaric atmosphere after beginning breathing seems, therefore, to cause oxidation of red blood cell membrane, denaturation of the membrane, inducing hemoglobin breakdown, and consequently hemolysis.