We describe the characterisation of a novel glycerophosphoglycolipid (termed lipid S) produced by Staphylococcus epidermidis grown in a chemically defined medium. Lipid S is a short chain length form of the cellular lipoteichoic acid (LTA). It shares common antigenic determinants with LTA, but its chain length of six glycerophosphate units contrasts with 40–42 units in LTA. Lipid S is exocellular and can be recovered from liquid growth medium whereas LTA is associated with the cell wall and membrane. Healthy individuals have low serum levels of IgG against lipid S, but significantly higher titres have been detected in serum from patients with central venous catheter-related sepsis due to coagulase-negative staphylococci and infection of orthopaedic prostheses. An indirect enzyme-linked immunosorbent assay test based on lipid S allows the rapid diagnosis of Gram-positive infection and may have clinical applications in the management of patients with sepsis.