The antibody response against Staphylococcus aureus antigens was quantitated in mice immunized with live staphylococci, the fibronectin-binding protein, clumping factor, or saline by means of enzyme-linked immunosorbent assay. Immunization with purified clumping factor induced a significant increase in antibody levels, while the fibronectin-binding protein gave a poor response. All mice immunized with live S. aureus developed a high antibody level. The immunization of mice with live S. aureus or clumping factor resulted in an increased resistance towards staphylococcal intraperitoneal infection.