The Phospholipase B of Liver

5. Adenosine diphosphate (ADP) was less inhibitory than ATP when it replaced part of the latter; inosine triphosphate had little inhibitory effect. Over a similar range, increasing concentration of uridine triphosphate produced progressively increasing inhibition. 6. No inhibitory action on the magnesiumactivated myofibrillar ATPase could be demonstrated with myokinase preparations. 7. Ethylenediaminetetraacetate strongly inhibited the magnesium-activated myofibrillar ATPase in concentrations which were only one-twentieth of that of the added magnesium chloride. Calcium activation of the ATPase was much less sensitive to ethylenediaminetetraacetate. 8. These findings are discussed in relation to the relaxation studies by other workers on glycerated muscle fibres.