517 the concentration of K+ or Na+ was 0.1 M or more. Also, the smaller amount of alkali required to adjust maleate buffer to pH 5.5 than to 7.9 or 7.0 may have augmented the inhibition reported by him at pH 5.5 because of the lower ionic strength; however, our results indicate, as do his, greater inhibition at acid pH values. The splitting of ATP by U02C12 may have had bearing upon his failure to detect inhibition at pH 7.9. This is especially likely if he used long incubations.1. It has been demonstrated that uranyl chloride ( UO2Cl2) inhibits myosin ATPase activity. The extent of inhibition is influenced by the strength of K-ion and is enhanced by calcium or magnesium. The pH value 6.6, of those tested, is most favorable for inhibition. 2 . Uranyl chloride per se Summary.causes dephosphorylation of ATP. This action is less than that caused by cerium and about equal to that of lanthanum.

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