Superoxide dismutase and glutathione peroxidase activities have been determined in newborns. Their mean values are approximately the same as in normal adults. In some cases a low content of superoxide dismutase and/or a high (superoxide dismutase/glutathione peroxidase) ratio are associated with hematological symptoms. In addition, a low superoxide dismutase activity is associated with hyperbilirubinemia and is present in two of the three cases showing maximal acetylphenylhdrazine-induced hemoloysis.