Increasing evidence suggests that activation of complement by AP contributes an important protective mechanism in early defense against bacterial invasion, before effective antibody levels are established. Since this is particularly important in children, we undertook a survey of AP in 83 normal children, ages 3 days to 15 years (x̄ = 54 months). There were approximately equal numbers of males and females, blacks and whites, and all children were free of fever and intercurrent infection. C3, Factor B, properdin,1 and β1H were quantitated by radial immunodiffusion with monospecific antisera; hemolytic function of AP was monitored by lysis of glutathione-treated human erythrocytes with inulin activated serum (N. Engl. J. Med. 286:180, 1973). Data were compared to normal adults. There was a wide range of values when compared to adults except for C3.