We have examined peripheral blood leucocytes (PBLs) from 17 multiple sclerosis patients, two patients with rheumatoid arthritis, one case of acute childhood measles and one case of subacute sclerosing panencephalitis, as well as 19 healthy adult controls for measles virus (MV) RNA, by the technique of reverse transcription-polymerase chain reaction. MV nucleocapsid gene specific primers were used to amplify all PBL-derived cDNA samples. These proved to be negative with the exception of the sample derived from the acute measles case. Selected cases were examined further, using fusion gene and matrix gene specific primers. MV RNA could not be detected.