The lengthening effect, in the presence of ATP, of myokinase preparations of varying degrees of purity is described. The preparations also depress the ATP-ase activity of the fibres to between one-fourth and one-fifth. The effect is activated by Mg ions (1 to 8 mM) and suppressed by Ca ions (0.2 mM) in the presence of Mg ions. It is shown that the magnitude of the lengthening effect and of myokinase activity are highly correlated under diverse circumstances. These effects cannot be distinguished from those of the original Marsh factor preparations, and it is therefore concluded that the main active principle is myokinase. The similarity, in ionic activation, between Marsh factor (myokinase) and pyrophosphate is discussed, and it is suggested that the latter may actually be produced by myokinase under the particular ionic conditions and in the presence of the muscle fibres.