The agar electrophoresis of the cerebrospinal fluid and the quantitative immune globulin determination contribute essentially to an assessment of the immunity state in inflammatory diseases of the CNS. 2. It is necessary to distinguish protein augmentation due to brain-barrier disturbances (accompanying proteinosis of the cerebrospinal fluid), which we find in metabolic and traumatic diseases, from a genuine increase in cerebrospinal fluid protein. 3. Autochthonous formation of cerebrospinal fluid protein can be demonstrated by means of quantitative IgG, IgA and gamma3-globulin determination.