A description is given of cerebrospinal fluid (CSF) immunoglobulins in 355 patients with demyelinating, infectious, neuropathic, and other neurologic disorders. An increase in the CSF IgG/albumin quotient was observed in 19/36 (53%) cases of definite multiple sclerosis (MS), in 13/47 (28%) cases of probable or possible MS, in 6/9 (67%) cases of proven herpes simplex viral encephalitis (HSVE), in 3/4 (75%) cases of neurosyphilis, in 1/1 case of subacute sclerosing panencephalitis (SSPE), in 2/9 (22%) cases of other central nervous system infections, and in 2/12 (17%) cases of polyneuritis when compared with a group of 236 patients having other neurological disorders. In contrast, a relative increase in the CSF IgA or IgM was seen only in some of the patients with central nervous system infections. It was also found that the quotient CSF/serum IgG, expressed as a percentage of the CSF/serum albumin, was better in distinguishing patients with definite or suspected MS from those with other neurological disorders than the quotients IgG/albumin or IgG/total protein. The CSF kappa/λ ratio and the CSF and serum complement fixing antibody titre to measles and herpes simplex virus were measured in many of the patients. In general, abnormalities of these measurements were associated with raised IgG/albumin quotients. However, in 8 patients with definite or suspected MS, a normal IgG/albumin quotient was found with abnormal CSF kappa/λ ratios (6 cases) or abnormal CSF titres of measles antibody (7 cases). In addition, two patients with HSVE had normal IgG/albumin ratios but detectable herpes antibody in the CSF. These findings suggest that the measurement of the relative concentration of CSF immunoglobulin in combination with the kappa/λ ratio and antibody titre to various viruses may supplement each other in the endeavour to detect central nervous system immunoglobulin synthesis in neurological diseases.