Aggregation of acetylcholine receptors (AChRs) on skeletal muscle fibers is thought to be mediated by the basal lamina protein agrin. Structural similarities shared by agrin and laminin suggested that the laminin receptor dystroglycan-alpha, part of a dystrophin-receptor complex, might also bind agrin. We show here that dystroglycan-alpha and dystrophin-related protein (DRP/utrophin) are concentrated within AChR aggregates in cultures of C2 myotubes and that agrin binds specifically to dystroglycan-alpha in in vitro assays. This binding is calcium dependent and is inhibited by monoclonal antibody (MAb) IIH6 against dystroglycan-alpha, heparin, and laminin, but not by fibronectin. In S27 cells, which do not aggregate AChRs spontaneously, agrin and laminin binding to dystroglycan-alpha are dramatically decreased. Moreover, MAb IIH6 significantly inhibits agrin-induced AChR aggregation on C2 cells. We conclude that dystroglycan-alpha is an agrin-binding protein and part of a dystrophin-receptor complex involved in AChR aggregation.