Aggregation of acetylcholine receptors (AChRs) on skeletal muscle fibers is thought to be mediated by the basal lamina protein agrln. Structural similarities shared by agrin and lamlnin suggested that the laminin receptor dystroglycan-a, part of a dystrophin-receptor complex, mlght also bind agrin. We show here that dystroglycan-a and dystrophin-related protein (DRPI utrophin) are concentrated withln AChR aggregates in cultures of C2 myotubes and that agrln binds specifically to dystroglycan-a in in vitro assays. This binding is calcium dependent and is inhlbited by monocionai antibody (MAb) llH6 against dystroglycan-a, heparin, and laminin, but not by fibronectin. in S27 cells, which do not aggregate AChRs spontaneously, agrin and laminin blnding to dystroglycan-a are dramatically decreased. Moreover, MAb llH6 significantly inhibits agrln-induced AChR aggregation on C2 cells. We conclude that dystroglycan-a is an agrin-binding protein and part of a dystrophin-receptor complex involved in AChR aggregation.