Sera and cerebrospinal fluids (CSF) from patients with subacute sclerosing panencephalitis (SSPE) were tested by indirect immunofluorescence for the presence of measles virus specific antibodies of the various heavy chain classes. IgG and IgA antibodies were detected in the CSF while IgG, IgM, IgA, IgD und IgE measles virus antibodies were found in a significant number of the patient sera. Sera from SSPE patients had slightly elevated levels of IgG, IgM, IgD and IgE while IgA was decreased. The heterogeneous heavy chain class distribution of measles antibodies suggests the possibility that non-complement fixing antibodies serve as blocking antibodies which aid in the persistence of intracellular measles virus infection in patients with SSPE.