In most invertebrates, creatine kinase is replaced by arginine kinase, which catalyzes reversibly the transfer of a phosphate group between adenosine triphosphate and arginine. In sea-urchin larvae, arginine kinase only is expressed whereas in adult sea-urchins both arginine kinase and creatine kinase can be found in the same tissue. In order to study their developmental regulation and properties, we have purified arginine kinase to homogeneity from the eggs of the sea-urchin Paracentrotus lividus. The purification involves ethanol and ammonium sulfate precipitations, followed by an anion-exchange chromatography, an affinity chromatography and a gel filtration. A 500-fold increase in specific activity leads to a specific activity of 360 IU/mg protein at 25 degrees C. Arginine kinase (pI = 5.7) is rapidly and irreversibly inactivated at 45 degrees C. Amino acid composition and Km values (2.08 mM for phospho-L-arginine and 1.25 mM for ADP) are also given. Determination of molecular mass by gel filtration and separation by SDS/polyacrylamide gel electrophoresis indicate that the enzyme is an 81-kDa dimer of two subunits of 42 kDa.