The paucity of data regarding phagocytosis in the newborn is reviewed. A method of evaluating phagocytosis in the human is presented, in which the proportion of "effective phagocytes" (cells containing 10 or more carbon particles) is scored, rather than the total number of cells engulfing particles. Employing the criteria described, we have found significant differences in phagocytic ability between the blood of premature infants and that of adults or full-term infants. Differences observed between full-term infants and adults in respect to phagocytic capacity of leukocytes were not clearly significant. In general, increasing phagocytosis was observed, approaching adult levels with increase in birth weight. Addition of fresh, normal adult serum to invitro preparations of premature infants' blood significantly enhanced phagocytosis. The substances in serum which stimulated phagocytosis were identified by electrophoresis to be in the fractions generally known as α1-, α2-, and β-globulin. A possible therapeutic use for adult serum in the premature is postulated and is being studied.