Sera of normal full-term infants, 2 weeks to 5 months of age, were analyzed for concentrations of calcium and inorganic phosphorus. Concentrations of calcium were not found to be significantly influenced by age, sex or type of feeding, the mean value being 9.6 mg/100 ml. Concentrations of inorganic phosphorus in sera of groups of infants receiving a variety of feedings were not well correlated with phosphorus content of the feedings. The mean concentration of inorganic phosphorus in sera of infants 2 weeks to 2 months of age was 7.0 mg/100 ml, approximately two-thirds of the values falling between 6.0 and 8.0 mg/100 ml. The mean concentration of inorganic phosphorus in sera of infants 2 to 5 months of age was 6.5 mg/100 ml, approximately two-thirds of the values falling between 5.8 and 7.4 mg/100 ml. Mean concentration of inorganic phosphorus in sera of boys was significantly greater than in sera of girls, particularly during the first two months of life. The importance of this observation is uncertain.