Report on quantitation of IgG in cerebro spinal fluid (CSF) and serum of 84 children at different ages. A relation was shown between the content of IgG in CSF and serum depending on age. The decrease of the ratio CSF-IgG/serum-IgG as well as of the total protein in the CSF during the first trimenon points out a post partal impediment of the diffusion from the blood to the CSF compartment. Beyond the first year of life the ratio of the concentrations mostly remains constant. Measurements in 135 children suffering from several neurological diseases showed that quantitation of IgG is of value only in case of suspicion of encephalitis with local synthesis of IgG.