Lumbar cerebrospinal fluid (CSF) from 30 cases of subacute sclerosing panencephalitis (SSPE) was studied for total protein and protein electrophoresis. Total CSF protein values were normal in almost all the cases, but the gamma-globulin was increased and had an oligoclonal morphology in all the cases. Absolute and relative values of some of the other electrophoretic fractions were decreased. Total protein and electrophoretic serum values in 25 cases did not differ significantly from the controls, although the gamma-globulin presented an oligoclonal morphology in 13 cases. CSF immunoglobulins were determined in 10 cases; IGG was increased in ll; IgA was traceable and slightly increased in only four cases; IgM was not found. These findings point to the intrathecal synthesis of IgG and the absence of barrier impairment. The decrease of the absolute values of some of the other CSF proteins suggests that homeostatic mechanisms for protein concentration in the CSF are maintained in SSPE.