Serum and cerebrospinal fluid (CSF) from 20 subacute sclerosing panencephalitis (SSPE) patients were analyzed with solid-phase radioimmunoassays designed to detect measles virus-specific IgM antibodies and IgM-class rheumatoid factor (RF). Serum and CSF from patients 1 to 16 did not contain measles IgM antibodies or elevated levels of RF. Serum from all and CSF from two of the remaining four patients were apparently measles IgM positive. RF levels were elevated, however, in serum from two and in serum and CSF from another of these four patients. Testing after removal of IgG or RF revealed that patient 17 had both measles IgM and elevated levels of RF in serum, and measles IgM in CSF. Reactivation of measles IgM synthesis was detected in the blood of patient 18. The serum of patient 19 and the serum and CSF of patient 20 contained only elevated levels of RF. The implications of these results are twofold. First, although measles IgM is found in some SSPE patients, it is not found consistently enough to be a marker for the chronic measles virus infection in these patients. Second, the possible interfering presence of RF must be taken carefully into account in any study of IgM antibody persistence.