Because of the possible involvement of group-specific component (Gc) or vitamin-D-binding protein in the immunological functions of mononuclear cells and the increased risk of central nervous system infections in early infancy, we studied Gc levels in the cerebrospinal fluid (CSF) of children. CSFs were examined for the Gc concentration using ELISA and rocket immunoelectrophoresis, with purified Gc as standard. The results showed a significant inverse correlation (p less than 0.05) between the age of the patients and CSF Gc levels. Gc levels in the CSF were significantly increased in infants less than 2 months of age (12.5 micrograms/ml), as compared to infants greater than 2 months (1.7 micrograms/ml, p less than 0.0028). In children greater than 2 months of age a significant correlation was found between Gc levels and those of other CSF proteins (albumin, IgG and total protein, p less than 0.002). However, no significant correlation between Gc levels and those of other CSF proteins was apparent in infants less than 2 months of age, indicating the possibility that the concentration of Gc in the CSF may be selectively increased in this age group.