The Lutheran blood group glycoprotein (Lu), also known as basal cell adhesion molecule, is an Ig superfamily transmembrane receptor for laminin alpha5. Lu is expressed on the surface of a subset of muscle and epithelial cells in diverse tissues and is thought to be involved in both normal and disease processes, including sickle cell disease and cancer. Here we investigated the binding of Lu to laminin alpha5 in vivo and in vitro. We prepared a soluble recombinant Lu (sol-Lu) composed of the Lu extracellular domain and a His(6) tag. Sol-Lu bound specifically to laminin-10/11 (alpha5beta1/beta2gamma1) in enzyme-linked immunosorbent assays and bound to bona fide basement membranes containing laminin alpha5 in tissue sections. Sol-Lu did not bind to tissue sections of laminin alpha5 knockout embryos, despite the fact that the four other alpha chains were present. To identify the Lu-binding site on laminin alpha5, we prepared modified alpha5 cDNAs encoding chimeric laminins containing all or part of the laminin alpha1 G domain in place of the analogous alpha5 regions. These constructs were used to generate transgenic mice. Proteins derived from transgenes were detected in basement membranes and were assayed for their ability to bind Lu by examining the localization of endogenous Lu and the binding of sol-Lu applied to tissue sections. Our results demonstrate that the alpha5 LG3 module is essential for Lu binding to laminin alpha5.