Abstract
Antigenic relationships among the matrix proteins of influenza A viruses were analysed using a competitive radioimmunoassay technique. There were at least two antigenic determinants on the M protein of A/PR/8/34 virus. One antigenic determinant was shared by several influenza A viruses of human, avian, swine or equine origin, while the second determinant was not necessarily shared by all influenza viruses since it was not detected on the M protein of A/chicken/Germany/N/49 virus. In addition, the cross-reactive determinant(s) of the M protein of N virus was not equally represented in other influenza A virus subtypes.