Cellular and humoral immunity was studied in 26 patients with subacute sclerosing panencephalitis. Results were compared with those of 14 normal controls and 11 patients suffering from other neurological disorders. It was shown that cellular and humoral immune responses are adequate in subacute sclerosing panencephalitis. The persistently elevated levels of serum immunoglobulin G (IgG) and IgA indicated a persistent infection, and their progressive rise in later stages correlated with the progressive nature of the illness. IgG progressively increased with the clinical stage in the cerebrospinal fluid unaccompanied by a corresponding rise in the measles antibody titer. This suggests that antigenic determinants other than those tested play a role in the production of IgG in the cerebrospinal fluid. The progressive increase in the ratio of cerebrospinal fluid to serum IgG with the advance of the disease suggests synthesis of IgG locally in the central nervous system. Elevated measles antibody titer in serum and cerebrospinal fluid is a consistent aid in the diagnosis of subacute sclerosing panencephalitis. It is more specific in cerebrospinal fluid than in serum. Its level did not vary significantly with the clinical stages or duration of illness. Depressed serum complement activity has been detected in some subacute sclerosing panencephalitis patients in whom serum levels of the third and fourth components of the complement were normal.