Bacterial infections are more frequent and severe in the first few days of life than at any time thereafter. It is also well recognized that in newborn infants clinical signs of infection may be minimal: accordingly, laboratory tests are of greatest importance for diagnosis in this period of life. Because of the difficulties in interpreting the meaning of granulocyte changes in preterm infants, we have examined the possible clinical usefulness of serum c-reactive protein, determined by a new, sensitive electrophoretic method (ZIA), for detection of bacterial infection in infants of this age. This study suggests that c-reactive protein testing is a helpful adjunct in diagnosing bacterial infection in the first month of life.