In a prospective study serum C-reactive protein (CRP) was measured in 100 premature infants. All babies were suspected of having bacterial infection (septicemia - meningitis) because of complications during pregnancy and/or during the perinatal period. CRP was measured with the radial immunodiffusion technique. 6/6 babies with bacterial infections proved by positive cultures from blood and/or CSF showed elevated levels of CRP already within 24 h after the first appearance of clinical symptoms suggesting sepsis. In 11 of 21 cases most probably suffering from septicemia CRP rose within a period of 82 h after the appearance of clinical signs. Even extremely immature infants were able to react with elevated CRP concentrations. Peak values of CRP were independent of birth weight. On the other hand, only 2 of 73 babies without clinical or laboratory findings of infection had slightly elevated amounts of CRP for a short time. Therefore it is suggested that increased levels of serum CRP are a valuable parameter for the early diagnosis of severe bacterial infections in premature infants.