A binding assay was developed and used to study the binding of oral streptococcus to immobilized human fibronectin, laminin, vitronectin, fibrinogen, heparin, and collagen IV. The protein binding was dependent on the broth used for bacterial growth. The binding after growth in brain heart infusion broth, trypticase soy broth, Todd-Hewitt broth, and Dulbecco's modified Eagle's medium was examined. Most of the strains were able to bind to immobilized fibronectin and laminin, and to a minor extent vitronectin. Binding was not observed on immobilized fibrinogen, collagen IV, or heparin. Measured surface hydrophobicity correlated well with the bacterial binding strength to the proteins. Streptococcal incubation with putative inhibitors indicates multiple binding mechanisms of a lectin-like and protein nature, possibly involving protein receptors.