The heavy chain of myosin subfragment-1 prepared by chymotrypsin treatment had a molecular weight of about 96 K. It was split into 26 K, 50K, and 21 K fragments on trypsin treatment. The effect of actin binding on the susceptibilities of the junctions between 26 K and 50 K and between 50 K and 21 K, and on that of alkali light chain 1 to trypsin was studied. The addition of actin increased the viscosity of the solution, and the apparent activity of trypsin decreased. We estimated this decrease as 35% by measuring the degradation of gamma-globin heavy chain, which is known not to interact with actin and subfragment-1 but is known to be susceptible to trypsin, in actin-subfragment-1 solution. Taking this value into consideration, we concluded that the 26 K-50 K junction became 5 times more and the 50 K-21 K junction became 3 times less susceptible to tryptic attack upon the binding of actin. We also observed that alkali light chain 1 became resistant to trypsin upon the binding of actin to subfragment-1. The relation between this conformational change in subfragment-1 and the cyclic interaction of subfragment-1 with actin and ATP is discussed.