Serum C-reactive protein determinations were done on 66 cord blood specimens and on capillary blood specimens of 669 apparently normal infants and 266 infants ill with known or suspected infection during the first 6 months of life. All but one of the cord blood specimens were negative for C-reactive protein. The incidence of positive reactors increased soon after birth with 50% of the normal newborns having positive reactions during the first week of life. This rapidly decreased after this period so that by 1 month to 6 months of age the incidence was only 2%. The intensity of the positive reaction in the majority was only up to 1 mm. Two-thirds of the infants with infection had positive reactions. The more severe the infection the greater the intensity of the positive reaction. C-reactive protein was also present in a variety of conditions besides infections. Serum C-reactive protein determination can be a valuable diagnostic aid in infections during the first 6 months of life when the usual laboratory criteria for infections are often not helpful. However, it has its limitations. Like any other single laboratory test, it must be used in conjunction with critical clinical judgment.