Variations of the main immunoglobulin fractions in CSF were studied in 79 children with various inflammatory processes of CNS (purulent and viral meningitis, post-infectious meningoencephalitis, polyradiculoneuritis, etc). This study was extended to a group of non-inflammatory neurological diseases. Increased values of CSF immunoglobulins (especially IgG and IgA) were found in the group of purulent meningitis and in the group of post-infectious meningoencephalitis. IgM was detected in very few cases. Generally, a correlation was observed between highest concentrations of CSF immunoglobulins and degree of meningeal inflammatory response, even if this was a component of other neurological diseases. It is suggested that the rise of CSF IgG and IgA may depend upon the CSF-blood barrier damage, as it occurs in most inflammatory diseases studied. On the contrary, a production "in situ" of CNS could explain the presence of cerebrospinal IgM.