A high molecular weight complex (sedimentation coefficient approximately 27 S) containing high-affinity binding site(s) for [(3)H]dihydrocytochalasin B has been isolated from a low ionic strength extract of human erythrocyte membranes by sucrose density gradient centrifugation. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis showed that actin, spectrin, and other minor components, including two polypeptides with the electrophoretic mobility of band 4.1, were present in the complex-containing fraction. Addition of this complex to a solution of muscle monomeric actin (G-actin) in a low ionic strength medium resulted in a rapid increase in viscosity to a level comparable to that of a solution of filamentous actin (F-actin). Electron microscopy showed that the viscosity increase reflected actin filament formation. The rate of induced actin polymerization was dependent on the amount of complex added to the G-actin; in less than 1 hr, less than 1 mug of protein from the complex-containing fraction induced the conversion of 0.4 mg of G-actin to the "F" from. Binding studies indicated that, upon polymerization of the actin, the cytochalasin binding complex became associated with the actin filaments. Low concentrations of cytochalasins D and E and dihydrocytochalasin B inhibited actin polymerization induced by the complex; the relative potencies of the drugs in inhibiting this process corresponded to their relative affinities for the complex, as well as their relative potencies in affecting cell motility. These results suggest that the cytochalasin binding complex functions as a regulatory site for cell motility by controlling formation and membrane attachment of actin-containing microfilaments in the cell.