Serum orosomucoid concentration was measured by laser nephelometry in 1970 serum samples collected from 1170 full term and preterm infants. The determinations were carried out in 1 h. Reference values are given: they show that the low levels at birth are influenced by gestational age. The concentrations increase rapidly during the first week in all infants, the adult values being reached by 10 months of age. High levels of orosomucoid concentration were detected in 85% of the infants with severe bacterial infections. Serum orosomucoid concentration proved less valuable in viral and parasitic infection. Twenty-six per cent of the sick infants without infection had a slightly elevated orosomucoid level which decreased rapidly. In the bacterial infections the evolution of serum orosomucoid concentration followed the clinical course. Thus serum orosomucoid concentration was a useful parameter for diagnosis and monitoring of bacterial infection in neonates.