Investigations on pathophysiological processes in the CSF space due to impairment of the blood-CSF barrier and due to an immune reaction are reported. Barrier disturbances and immune reaction are demonstrated by means of absolute values of CSF protein electrophoresis and by measurement of immunoglobulins. With inflammatory diseases of the CNS, an altered blood-CSF barrier will often be seen at the moment of the first lumbar puncture. Multiple sclerosis, subacute leucoencephalitis and neurosyphilis show signs of a distinct immune reaction together with an increase of the γ-globulins as well as the immunoglobulin G in the absence or slight presence of a barrier impairment. Non-inflammatory diseases of the CNS have variable barrier disturbances.