Abstract Earlier investigations extended exclusively to comparisons of the red blood cells (RBC) Cu,Zn-superoxide dismutase in neonates of various ages. We have now carried out comparative measurements on the activities of RBC catalase and glutathione peroxidase from the same source. Besides the-enzymes, we also measured the lipid peroxidation (LP). The differences between the RBC of premature and full-term neonates were found to be significant. The lower antioxidant enzymatic activity and LP level in premature infants are explained by the immaturity of the biochemical systems and the different lipid composition of the RBC membranes.