The presence of A-protein was sought in strains of S. epidermidis and S. aureus, sensitive or resistant to methicillin. The technique used was that of passive hemagglutination of red cells of sheep, either normal or formolated and sensitised by anti-erythrocyte antibodies. A-protein was not found in any of the 157 strains of S. epidermidis. It was found in 82.5 p. cent of 360 strains of S. aureus. The methicillin resistant strains seemed less rich than the protein A-sensitive strains, which was detected more frequently when formolated red cells were used. The interest of this test for bacteriological diagnosis is discussed.