The normal microbial colonization of sites in the body's tissues by certain bacteria requires that the bacteria first bind to extracellular secreted constituents, cell-surface membranes, or cell matrixes. This study examines two interactions of a variety of bacteria with the cell matrix noncollagenous proteins fibronectin and laminin and with basement membrane (Type IV) collagen. Adherence of bacteria to matrix proteins coated on tissue culture wells was examined with the use of radiolabeled bacteria. Staphylococcus aureus, Streptococcus pyogenes, and Streptococcus sanguis bound well to fibronectin, laminin, and Type IV collagen, whereas a variety of gram-negative organisms did not bind. The interaction of soluble laminin, fibronectin, and Type IV collagen with bacteria was monitored by nephelometry with the use of a platelet aggregometer. S. aureus aggregated in response to fibronectin, laminin, or Type IV collagen. In contrast, gram-negative organisms did not aggregate with these proteins. It appears that fibronectin, laminin, and Type IV collagen can bind and aggregate certain gram-positive bacteria, and this binding is dependent on the surface characteristics of the organism. These adhesion molecules may play a role in the normal colonization of sites by microorganisms and in invasion during infections.