Complement fixing (CF) antibody titers to measles, parainfluenza (PI) types 1 and 3, mumps, herpes type 1, and cytomegalovirus (CMV) in serum and cerebrospinal fluid (CSF) of 33 patients with subacute sclerosing panencephalitis (SSPE) were evaluated. Results were analyzed in comparison to 11 patients with neurological diseases other than SSPE and 7 normal subjects. All SSPE patients had elevated serum and CSF measles antibody titers. The number of SSPE patients manifesting elevated titers other than measles did not reach statistical significance when compared to controls, except for PI type 1. This suggests a possible dual infection with measles and PI in SSPE. The anticomplementary effect detected in the serum and CSF of some patients indirectly suggests the presence of immune complexes.