Eleven strains of Staphylococcus lugdunensis from different clinical sources were investigated for their ability to bind125I-labelled collagen (Cn) type I and IV, fibronectin (Fn), vitronectin (Vn), laminin (Lm), fibrinogen (Fg), thrombospondin, plasminogen (glu- and lys-form) and human IgG. All the strains bound these proteins, although a higher degree of binding was obtained for Cn types I and IV and IgG with mean values of 36%, 32% and 26% binding, respectively. In tests with proteins immobilised on latex beads in a particle agglutination assay, eight of the 11 strains bound Cn type I and seven bound Fg, whereas no strain bound immobilised IgG. Binding to immobilised Cn-I, Fg, Lm and Vn was abolished when the bacterial cells were treated with proteases or heat, indicating cell-surface receptors with protein characteristics. Cell-surface extracts of S. lugdunensis 2342 were able to totally inhibit binding of the homologous strain and S. aureus Cowan 1 to latex-immobilised proteins Cn-1, Lm, Vn, Fn and Fg. The binding of125I-labelled Cn IV by S. lugdunensis 2342, was heat sensitive, whereas the binding to S. aureus Cowan 1 was heat resistant. The strains gave negative results in tests for the presence of protein A with a S. aureus protein A gene probe and with sensitised red blood cells. No production of heat-stable nuclease (TNase) could be detected by monoclonal antibodies against TNase or by the polymerase chain reaction with an oligonucleotide sequence from S. aureus TNase as primer. When the cell surface characters of the S. lugdunensis strains were studied, five were found to be hydrophobic and negatively charged, four hydrophilic and positively charged and two hydrophobic with positive net charge.