Coagulase-negative staphylococci produce an exocellular glycolipid antigen which has potential as a serological marker of infection in bone. The value of this newly detected antigen was investigated by enzyme-linked immunosorbent assay (ELISA) in 15 patients with culture-proven infection of prostheses caused by Gram-positive bacteria. The antigen was purified by gel-permeation chromatography from the culture supernatants of coagulase-negative staphylococci grown in a chemically defined medium. There were significant differences (p < 0.0001) between the serum IgG and IgM levels in patients with infection due to Gram-positive staphylococci and those of a control group of 32 patients with no infection. The ELISA test, which has potential for the diagnosis of infection, may be valuable in distinguishing between staphylococcal infection around prostheses and aseptic loosening.