A calmodulin-binding protein called "caldesmon" was purified from chicken gizzard muscle as the major calmodulin-binding protein in this tissue. Its molecular weight estimated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis was 150,000, and two of these polypeptides constituted the native molecule. Caldesmon is an actin-binding protein also, binding F-actin reversibly in the presence or absence of Ca2+. The interaction of caldesmon with F-actin was abolished by the binding of calmodulin with the caldesmon. Because the interaction between caldesmon and calmodulin was Ca2+-dependent but the interaction between caldesmon and F-actin was not, Ca2+ acts as a flip-flop switch between the formations of two complexes, caldesmon.calmodulin and caldesmon.F-actin: increasing the formation of the former complex at increased Ca2+ level and the formation of the latter complex at decreased Ca2+ level. The equilibrium of the formations of both complexes was achieved at a Ca2+ concentration near 1 microM.