The preparation and properties of 1: 1 complexes of aquochromium (III) with adenosine-S'-cliphosphate (ADP) and -triphosphate (ATP) have recently been reported [l, 21, together with their behaviour towards a number of phosphoryl transfer enzymes normally utilizing Mg*+-ADP or -ATP. In view of the inhibition reported for aquochromium (III)-ATP with hexokinase, it has been suggested that these complexes may be useful inhibitors for kinetic analysis [Z]. Their potential utility, however, is lessened by the instability, attributable to o&ion, of aquochromium (III) complexes in the neutral pH region in which phosphoryl transfer enzymes optimally function. In an attempt to circumvent this difhculty and to obtain a clearer picture of the mode of interaction between chromium (III) nucleotide complexes and kinase enzymes, n-e have e xamined the interaction be-Ween creatine kinase (rabbit muscle) and a tetraaminechromium (III)-ADP complex stable at neutral pH, and also between creatine kinase and aquochromium (III)-ADP under acid conditions. The interaction between creatine kinase and metal-nucleotide complexes has been extensiveiy studied by kinetic [3] and magnetic resonance [4] techniques, and current thinking on the role of the divalent metal ion in kinases [a] suggests ths,t an enzyme such as creatine kinase with a substrate bridge arrangement of the ternary enzyme-nucleotide-metal complex (in contrast to the metal bridge arrangement present in such enzymes as pyruvate kinase) offers the best opportunity for the substitution-inert chromium (III) complex to bind at the active site. Further, creatine kinsse is relatively stable towards precipitation in acid solution [S], making it amenable to studies at pH values where problems of oh-&ion with aquochromium (III) complexes are --1 Cr(er&ADP was synthesized by heating 10 m&I cis-[Cr(en)n (dmf)l]