Zusammenfassung Polyelectrolytes as interaction inhibitors and the significance of Ca and Mg in actin-myosin interaction 1. 1. Evidence is given that there are interaction inhibitors as well as myosin ATPase poisons among the high molecular electrolytes. 2. 2. It is shown that Ca-activated actomyosin in both the gel and the sol state splits ATP quantitatively with the same speed with which pure L-myosin under the same conditions splits ATP if the splitting by actomysin is related to the unit of weight of the L-myosin component that is contained in the actomyosin. 3. 3. The rate of Ca-activated ATP splitting is likewise identical in actomysin and L-myosin systems if myosin ATPase poisons or interaction inhibitors are present. In the present of myosin. ATPase poisons the splitting in both systems decreases to the same extent. In the presence of interaction inhibitors the splitting in both systems is not affected at all. 4. The observations 2 and 3 lead to the conclusion that the Ca-activated ATP splitting—even in actomyosin gels—is always based on the action of the L-myosin ATPase. Only the presence of Mg brings about in actomyosin gels an enzymic interaction between actin and myosin. 5. 4. The interaction inhibition by protamine and polyethensulfonate is not based on an increase in the ionic strength and is independent of the protein charge in the pH range from 6 to 9. 6. 5. The interaction inhibitions by means of the polyanion polyanetholsulfonate and the polycation protamine can be shown to be the same for all concentrations of polyions if the inhibition is related to the amount of polyions that is bound by actomyosin instead of to the amount of polyions added. 7. 6. The polycation protamine is preponderantly bound by F-actin, whereas the polyanion polyethensulfonate is almost exclusively bound by L-myosin. 8. 7. When high polymer interaction inhibitors are joined with contractile proteins they dissociate less rapidly and completely than do low molecular inhibitors and the proteins. It is shown that also the binding of polyelectrolytes to actomyosin is reversible.