Certain methicillin-resistant Staphylococcus aureus strains contain a 230-kDa cell-wall protein which is not present on the surface of other staphylococci. The presence of this 230-kDa protein is associated with a negative test result in commercial assays designed to detect fibrinogen-binding proteins and/or protein A on the staphylococcal surface. We have purified and partially characterised the 230-kDa protein from a lysostaphin digest of a non-agglutinating methicillin-resistant S. aureus strain. Partial amino acid sequence data obtained from the purified protein did not reveal any significant similarities to known proteins which indicates that the protein is novel. The 230-kDa protein was very sensitive to proteolysis; soluble plasmin, or plasmin formed on the bacterial-cell surface, rapidly degraded the 230-kDa protein to a 175-kDa form. The finding that the 230-kDa protein bound to lectins allowed its purification by affinity chromatography on immobilised wheat germ agglutinin. Furthermore, the degradation of the 230-kDa protein was associated with an increased adherence of non-agglutinating methicillin-resistant S. aureus cells to solid-phase fibronectin, fibrinogen or IgG.