The interaction of Actinoabacillus actinomycetemcomitans with the basement membrane protein, laminin, was examined in a 125I-labeled protein-binding assay. The binding of laminin increased by lowering the pH. The ability to bind laminin was decreased in cells at the stationary phase of growth and by the presence of blood in the culture medium. Laminin binding to this bacterium was saturable, and the affinity constant was 4.6 nM. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis and Western blot (ligand blot) analysis of cell-envelope and outer membrane of A. actinomycetemcomitans displayed a 125I-laminin-reactive protein band with a molecular weight of 29 k. The laminin-binding protein was the previously described outer membrane protein A of A. actinomycetemcomitans. It was identified by its heat-modifiable property, detergent-solubility profile and reactivity with outer membrane protein A-specific polyclonal antiserum. At acidic pH, 25I laminin bound to several cell-envelope components of A. actinomycetemcomitans, but at neutral pH, laminin bound only to the heat-modifiable protein. Despite the existence of the laminin-binding protein, cells grown in blood-containing media did not bind laminin. Several mammalian proteins interfered with laminin-bacterial interaction, including lactoferrin, which binds to the same bacterial protein that inhibited and displaced the laminin-bacterial interaction.