Polyacrylamide-gel disc electrophoresis has been applied to the fractionation of CSF samples obtained from 2m control subjects (range of age 16 to 77 yr, mean 41.0 yr) and from 157 patients with various neurological diseases, with the purpose of evaluating the diagnostic significance of separated protein bands. It was found that the immunoglobulins, with some exceptions, were fractionated in the G-zone which was defined as an area between the origin and 0.57 in terms of relative migration based on transferrin (RT). Fourty-four percent of the whole Bence Jones protein bands which were examined were detected in the area of RT 0.70-0.90. The normal values of CSF protein fractions were (means+/-SD):Pre-alb-zone, 10.97+/-2.31%; Alb-zone, 40.74+/-5.72%; A1-zone, 5.25+/-0.72%; A2-zone, 8.23+/-1.24%; B1-zone, 10.82+/-2.18%. B2-zone, 7.94+/-1.93%; G-zone, 16.05+/-2.49%. The following significant results were obtained: (1) When the total protein concentration (TP) of CSF is increased: the percentage of the Pre-alb-zone is lowered and that of the A1- and G-zones is increased in diseases such as neurosyphilis, infectious meningitis, brain tumour, multiple sclerosis, intervertebral disc protrusion and cerebral infarction. The A2-zone was diminished in cerebral infarction, and a similar change was seen in brain tumour with in addition a decrease of the B2-zone; (2) When the TP was normal: the percentage of the G-zone was increased in multiple sclerosis, brain tumour and intervertebral disc protrusion. In the latter 2 disorders the A1-zone also increased. This fraction also increased in the cases of cerebral infarction, whose mean age was, however, much higher than that of the control subjects.