Actin purified by a new, simple, and rapid purification procedure activated the ATPase activity of both heavy meromyosin and Subfragment 1 of heavy meromyosin, and this activation was not inhibited by the removal of Ca²⁺. Preparations of tropomyosin-troponin inhibited (by 85%) both the acto-heavy meromyosin and acto-Subfragment 1 ATPases in the absence of, but not in the presence of, Ca²⁺. This inhibition was shown to result from binding of the tropomyosin-troponin complex solely to actin and in a ratio of about 1 mole of tropomyosin-troponin to 7 moles of actin.