A radioimmunoassay (RIA) for quantitating IgG, IgA and IgM in unconcentrated CSF has been developed. The amounts and percentages of these immunoglobulins in CSF from 31 normal individuals were determined. Using these values as normal, CSF from patients with syphilis, encephalitis, subacute sclerosing panencephalitis (SSPE), and multiple sclerosis (MS) was studied. Abnormalities were detected, indicating the potential relevance of more extensive study of the CSF immunoglobulins. CSF from patients with myotonic dystrophy and myasthenia gravis was normal. RIA was compared with rocket electroimmunodiffusion (EID) for the quantitation of IgG. Although RIA consistently gave lower absolute values, both assays reliably detect elevated IgG in CSF. However, an advantage of RIA is its capacity to quantitate IgA and IgM.