Phase reactants were evaluated prospectively in babies suspected of having sepsis. Among 318 babies, there were 22 proven and 10 "very probable" cases of neonatal sepsis. Of the proven cases 14 survived and 8 died. The survivors had a positive latex C-reactive protein (CRP) in 11 cases and an alpha1-acid glycoprotein (AGP) level greater than 0.5 g/l in 12 cases. Among those who died, one had a positive latex CRP and none had AGP greater than 0.5 g/l. These findings were supported by positive CRP and elevated AGP in almost all "very probable" cases, all of whom survived. These data in newborn infants support the hypothesis that acute phase reactants have a functional role in combating infection.