The Yersinia pseudotuberculosis invasin protein promotes bacterial penetration into mammalian cells by binding to several beta 1 chain integrins. We show here that proteins containing the cell-binding domain of invasin bind to the fibronectin receptor alpha 5 beta 1 isolated from human placenta and immobilized on a filter membrane. Two forms of the receptor, each having a molecular weight of about 290,000, were immunodepleted by monoclonal antibodies specific for the beta 1 subunit or the alpha 5 beta 1 heterodimer. The binding of invasin to the receptor immobolized on the filter, or to whole JAR cells, reaches saturation after 90 min and has an apparent dissociation constant (Kd) of 5.0 x 10(-9) M. Invasin binding to alpha 5 beta 1 is inhibited by the 120-kDa chymotryptic fragment of fibronectin in a competitive manner with an inhibition constant (Ki) of 7.5 x 10(-7) M. Furthermore, invasin-receptor binding is also inhibited by the hexapeptide GRGDSP, and monoclonal antibodies that block cell attachment to invasin-coated surfaces also block cell attachment to fibronectin-coated surfaces. These results indicate that invasin and fibronectin bind to the same, or closely located sites on alpha 5 beta 1, although invasin binds with a much higher affinity than does fibronectin.