Changes in serum concentrations versus time of C reactive protein and orosomucoid were investigated in 134 neonates with either materno-foetal infection (group A, n = 111) or nosocomial infection (group B, n = 23). Both proteins were significantly elevated in group A neonates, with mean +/- S.E.M. values of 0.029 +/- 0,003 g/l for C reactive protein (n = 111) and 0.56 +/- 0.003 g/l for orosomucoid (n = 37). In cases with superinfection serum levels of both proteins rose before clinical symptoms developed. Changes in orosomucoid concentrations occurred a few hours after changes in C reactive protein concentrations. Monitoring C reactive protein and orosomucoid levels, in an excellent means of assessing the effectiveness of treatment and of deciding on its withdrawal when both proteins return to normal values.