In 312 preterm and term newborn infants, serum concentrations of C-reactive protein (CRP), haptoglobin and .alpha.1-antitrypsin were measured during several days by radial immunodiffusion. White blood cell count and the ratio of band to total neutrophils (B/N-ratio) were determined. In 12 infants with proven sepsis CRP was elevated above the upper limit of the normal range (20 mg/l). Successful therapy was followed by a decrease of CRP concentration. In infants with suspected infection, high CRP values were found in most cases. Haptoglobin and .alpha.1-antitrypsin concentrations did not differ significantly between the groups of infants without infection, with proven infection and with suspected infection. White blood cell count and B/N-ratio were not appropriate for the early identification of bacterial infection in the newborn period.