The spectrin-based membrane skeleton, an assembly of proteins tightly associated with the plasma membrane, determines the shape and mechanical properties of erythrocytes. Spectrin, the most abundant component of this assembly, is an elongated and flexible molecule that, with potentiation by protein 4.1, is cross-linked at its ends by short actin filaments to form a lattice beneath the membrane. These and other proteins stabilize the plasma membrane, organize integral membrane proteins and maintain specialized regions of the cell surface. A membrane-skeleton-associated calmodulin-binding protein of erythrocytes is a major substrate for Ca2+- and phospholipid-dependent protein kinase C (ref. 5), and thus is a target for Ca2+ by two regulatory pathways. Here we demonstrate that this protein, called adducin: (1) binds tightly in vitro to spectrin-actin complexes but with much less affinity either to spectrin or to actin alone; (2) promotes assembly of additional spectrin molecules onto actin filaments; and (3) is inhibited in its ability to induce the binding of additional spectrin molecules to actin by micromolar concentrations of calmodulin and Ca2+. Adducin may be involved in the action of Ca2+ on erythrocyte membrane skeleton and in the assembly of spectrin-actin complexes.