Incubation of zymosan particles with serum was shown to be accompanied by their partial breakdown into the oligosaccharides maltose, maltotriose, and maltotetraose, that were quantified under the form of glucose after degradation by alpha-glucosidase. Incubation with normal adult serum yielded twice as much glucose than incubation with cord serum. The degradation of zymosan could be mimicked by incubation with purified pancreatic alpha-amylase. A correlation was also found between the amount of oligosaccharides released from the particles during their incubation with serum and the concentration of amylase in the sera. The latter is low in newborns and increases several-fold, to nearly adult levels, at 3-6 months of age. Preincubation of zymosan with purified pancreatic alpha-amylase resulted in an approximately 50% increase in the binding of radioactive anti-C3c in the supernates obtained after subsequent incubation with cord serum but not with adult serum. With six of 24 cord sera, but not with adult serum, it also resulted in an augmentation of the chemiluminescence accompanying the phagocytosis of the particles after their opsonization. It is concluded that the relative inefficiency of cord and newborn serum in opsonizing zymosan is attributable not only to their lower content of the components of the alternative pathway of complement, but also to a lower concentration of serum amylase.