Attachment of bacteria to the host tissue is regarded as a crucial step in the development of many types of infections. Recent studies by us and others have shown that matrix proteins which serve as adhesion proteins for eucaryotic cells may also be recognized by some bacteria. In the present communication, we report that several strains of viridans streptococci are able to bind to laminin. Most strains isolated from blood and heart valves of patients with endocarditis expressed laminin receptors, whereas only a few of the strains isolated from the oral cavity recognized this protein. This observation indicates that laminin binding might be an important factor in the pathogenesis of viridans endocarditis. Laminin binding to two strains (Streptococcus mitis UAB594 and UAB597) isolated from patients with endocarditis was characterized further. The bacterial cells expressed a limited number of laminin receptors (4 X 10(2) to 1 X 10(3) per cell) which bound the protein in a high-affinity interaction (Kd, 40 to 80 nM). This receptor of S. mitis UAB594 was heat labile and could be solubilized from bacteria by brief digestion with trypsin. Solubilized receptors which competed with cell-bound receptors for 125I-laminin could be adsorbed on laminin-Sepharose but not on Sepharose substituted with fibrinogen or fibronectin. Comparison of laminin receptors from S. mitis with those previously described for Streptococcus pyogenes suggest that different sites in the laminin molecule are recognized by the two bacteria and hence that the corresponding receptor molecules are not identical.