Four monoclonal antibodies to the h1 agglutinogen were produced by conventional means, and slide agglutination of S. aureus type strains was performed with protein A affinity purified IgG1 antibodies. In accordance with Oeding's serotype system the type strains 17A and 670 were strongly and consistently agglutinated. In addition, however, several of the remaining twelve type strains investigated showed varying reaction patterns. Our results indicate that the h1 agglutinogen may be more widely distributed among S. aureus strains than previously assumed.