Binding was measured in the presence of 2.5 μm smooth muscle myosin and varied concentrations of [14C]iodoacetamide-labeled caldesmon. The conditions used were: 30 mm NaCl, 5 mM MgCl2, 10 mm imidazole-HCl (pH 7), 1 mm ATP, 1 mm dithiothreitol, at 25 °C. The theoretical curve describes a single class of binding sites with an association constant of 9.9 × 105 m−1 and a stoichiometry of 1 caldesmon molecule/0.92 myosin molecules.