The synthesis of measles virus polypeptides in cells in their fifth passage after removal from the brain of a patient with subacute sclerosing panencephalitis (SSPE) has been investigated using immunoprecipitation techniques. Although the other major virion polypeptides were found, synthesis of the M polypeptide was not detected. High titers of antibodies to the other virion proteins were found in the cerebrospinal fluid, as well as the serum, of a patient with SSPE, but only a trace of antibody to the M polypeptide was found at the lowest dilutions. These results, and the earlier finding of the lack of antibody to M in the sera of many patients with SSPE, support the previous suggestion that there is decreased synthesis of the M polypeptide in the cells of the brain in patients with SSPE. © 1979.