An enzymic synthesis utilising arginine kinase for preparing arginine phosphate in a high state of purity is described. The dissociation constant of magnesium arginine phosphate, determined by gel filtration, was 30.0 +/- 0.9 mM. That for potassium arginine phosphate was calculated to be 63.0 +/- 4.0 mM measured by the effect of potassium on the apparent magnesium dissociation constant. The effect of KCl on the reaction catalysed by the type-M pyruvate kinase from Carcinus maenas (the common shore crab) pincer and leg muscle was investigated. No effect was seen on the C. maenas pyruvate kinase activity, apart from that due to alteration of the K+ concentration, on adding up to 70 mM potassium arginine phosphate to the reaction medium. The less pure form of arginine phosphate was found to give an apparent noncompetitive inhibition of the enzyme when phosphoenolpyruvate was the varied substrate. This apparent inhibition can be accounted for by the removal of ADP from the assay medium in a side reaction involving arginine kinase and arginine phosphate. These results are discussed in terms of the possible physiological control of the type-M pyruvate kinase from C. maenas.