Three proteins of the "acute phase" (p.a.p.) were determined in the serum of 113 premature and newborns during the first 10 days of life. In all 13 cases with septicemia or severe infection high levels of c-reactive protein (CRP) and haptoglobin (Hp) were found, in 22 babies with suspected infection elevated CRP- and Hp-concentrations were found 13 and 14 times respectively. In 73 healthy newborns low levels of CRP were detectable 6 times, Hp however, was markedly elevated 25 times. Nutrition or injuries did not influence the production of Hp. The concentration of the third p.a.p. the acid alpha 1-Glycoprotein, in serum of sick and healthy neonates did not differ markedly. The indirect proof of bacterial infections by the p.a.p. is possible even in prematures under 1000 g and generally soon after the contact with pathogenes, for example in the cordblood after a previous rupture of membranes. Since CRP is the only p.a.p., that is not increased by transfusion or by age, it is by far the most suitable of the three examined p.a.p. for the detection of bacterial infections in newborn.