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. In addition 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 found 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. In contrast, haptoglobin and alpha 1-antitrypsin concentrations differed not significantly between the group of infants without infection, with proven and with suspected infection. White blood cell count and B/N-ratio also were not appropriate for the early identification of bacterial infection in the newborn period.