Studies on the interactions of staphylococci with fibrinogen, fibrin split products, and prothrombin have formed the basis for the clumping tests for coagulase and fibrin degradation products. We investigated the role of another circulating protein, fibronectin, in clumping Staphylococcus aureus. Fibronectin is a dimeric glycoprotein with high molecular weight that is present in both blood and tissue and is involved in opsonization, clotting, healing of wounds, cell-to-cell attachment, and differentiation. Each fibronectin molecule has two S. aureus binding sites, thus allowing lattice formation. We defined conditions under which fibronectin will cause agglutination of S. aureus. Strains of S. aureus that were most easily clumped had the largest number of fibronectin receptors. Trypsinization or gentle sonication removed the fibronectin binding and agglutinating receptors from S. aureus. These treatments did not alter viability, which suggests that binding is a superficial component of the organisms. Invasive fibronectin-binding strains were from a wide variety of phage types. Twenty two S. aureus isolates from patients with invasive disease were more readily agglutinated and had a greater number of fibronectin binding sites than 19 noninvasive strains (p less than 2.5 X 10(-4)). This suggests that the pathogenicity of S. aureus invasion may be enhanced by binding of bacteria to tissue fibronectin or by agglutination of bacteria by plasma fibronectin. Thus, the fibronectin receptors on S. aureus that mediate agglutination might also permit invasion of host tissues.