AbstractThe myosin molecule was extracted from the smooth muscle parts of horse esophagus and purified by ammonium sulfate fractionation. The schlieren pattern of the sedimentation velocity run showed a very sharp single peak of.5.9. S (s20,w). Molecular weight of the protein was measured by means of the Archibald and sedimentation equilibrium methods, both in 0.5MKCI buffered by 1/150Mphosphate at pH 7.5 and at 5°C. The values obtained were 6.25 × 105and 5.81 × 105respectively, for the two methods. The second virial coefficients were 1.1 × 104and 1.2 × 10−4ml/g. Denatured smooth muscle myosin was prepared in a solution of 5Mguanidine HC1 containing 0.4MKC1 and 0.2Mβ‐mercaptoet hanol buffered at pH8.0. The weight‐average molecular weight of the denatured smooth muscle myosin was 2.24 × 105and the second virial coefficient was 7.6 × 10−4ml/g. The values described above are in good agreement with those reported for rabbit skeletal myosin with ammonium sulfate fractionation. The molecular dimension of the molecule is estimated as the value for an axial ratio of 100, assuming a rigid rod molecular model for this molecule, both the thermodynamical and hydrodynamical treatment being in a good agreement with