A radioimmunoassay (RIA) which depends on the property of protein A of Stapbylococcus aureus to combine with the Fc-fragment of immunoblobulins was developed. This technique was employed to measure antibodies in human and various animal sera. It coulb be demonstrated that the staphylococcal RIA was at least as sensitive as the previously decribed radioimmunoprecipitation technique in detecting antibodies to M.pneumoniae in human sera. In addition, antibodies to M. pneumoniae could be demonstrated in sera of hamsters intranasally inoculated with the organisms. Antibodies could also be demonstrated in rabbit sera after immunization with M.pneumoniae. The test proved to the considerably more sensitive than conventional tests for detection of antibodies to the organisms. The test requires only small amounts of reagents and is relatively inexpensive.