The evolution of the main serum opsonins in neonates and infants of varying gestational age was investigated to provide reference values for these opsonins in early infancy. Serum concentrations of immunoglobulins, IgG subclasses, C3, C4 and fibronectin were serially measured from birth until the age of 6 months in term and preterm infants. Measurements were performed by rate nephelometry. Five hundred and sixty six neonates (gestational age 26-41 weeks) were examined at birth, 233 at 1 month, 218 at 3 months, and 147 at 6 months, respectively. The same measurements were performed in 54 pairs of neonatal/maternal samples and in 230 apparently healthy adults. Gestational age had a significant impact on serum IgG, IgG subclasses, C3 and C4 up till the third month, and on fibronectin until the first month. No such impact was observed for IgA and IgM. Sixteen per cent of the neonates had IgM concentrations higher than 0.2 g/l at birth, suggesting that the critical concentration of serum IgM at birth for suspected intrauterine infection should be reconsidered. Concentrations of all opsonins at birth were significantly lower than adult reference values. They only approached or even reached adult values by the third or the sixth month. Data from analysis of the neonatal and the corresponding maternal sera indicate that there is a preferential active transplacental transport of IgG subclasses in the order of IgG1, IgG3, IgG2 and IgG4. These results show that concentrations of immunoglobulins, C3, C4 and fibronectin undergo changes during the first months of life, depending not only on the infants' postnatal age but also on gestational age.