Concentrations of immunoglobulin G (IgG) and albumin in cerebrospinal fluid (CSF) and serum were measured in 118 "normal" Chinese children from birth to 15 years old to obtain the reference values for the evaluation of the blood-brain barrier (BBB) function. The values of the CSF/serum IgG ratio, CSF/serum albumin ratio and IgG index (IgG ratio/albumin ratio) were then calculated for each subject. A relation was found between the content of IgG in CSF and serum depending on age. The gradual decrease of CSF IgG and IgG ratio during the first 6 months indicated a postnatal impediment of the diffusion of IgG from the blood to the CSF. In the meanwhile the CSF/serum albumin ratio continued to drop until the age of one year. The slower slope of the decrement of the latter ratio further demonstrated the differential permeability of the BBB in favor of the shift of small molecular protein. The change of the protein permeability within the first year of life shows a characteristic of the maturational process of the BBB. Nevertheless, IgG index has no age-dependent variation throughout the childhood period. It remains constant in the absence of inflammation within the central nervous system.