Using a direct enzyme-linked immunosorbent assay (ELISA), we examined serum and cerebrospinal fluid (CSF) from six patients with subacute sclerosing panencephalitis (SSPE) and control subjects for presence of measles-virus-specific IgM antibodies. All samples from the SSPE patients contained demonstrable titers of measles antibodies. The levels of measles IgM antibodies were higher in CSF diluted 1:5 than in serum diluted 1:50, reflecting a local production of IgM antibodies in the central nervous system. Antibody titers remained constant over the course of SSPE in three of the patients followed for three to six months. The IgM ELISA had high sensitivity as well as specificity and was not complicated by false-positive reactions owing to the presence of rheumatoid factor.