We have investigated Pseudomonas aeruginosa binding to plasma and cellular fibronectin (FN), in both their soluble and insoluble forms. Bacterial binding to insoluble FN was studied by exposing coverslips coated with FN to radiolabeled microorganisms. P. aeruginosa binding to soluble FN was investigated (1) by comparing radiolabeled bacteria treated with FN with PBS-treated bacteria in their adhesion to a collagen matrix; (2) by analyzing the reactivity of P. aeruginosa with plasma or cellular FN adsorbed to gold particles with transmission electron microscopy (TEM). P. aeruginosa did not bind significantly to insoluble plasma or cellular FN, or to soluble plasma FN. In contrast, bacterial treatment with soluble cellular FN significantly increased the adhesion to the collagen matrix. With TEM, we confirmed the reactivity of P. aeruginosa with soluble cellular FN. Because there is a marked secretion of cellular FN during wound repair, we speculate that this reactivity may account for the propensity of P. aeruginosa to infect repairing tissues.