Adenylate deaminase of Porphyra crispata was purified to an electrophoreitically homogeneous state. The N-terminus of the enzyme protein was found to be alanine. The amino acid composition of the protein was also obtained.?It was suspected that the enzyme could be a glycoprotein. Analyses by disc electrophoresis with and without sodium dodecyl sulfate and molecular sieve?chromatography showed that the monomeric form of the protein with an approximate molecular weight of 7 x 10 4 could associate reversibly to form?oligomers. Kinetic data indicated, however, that the association of the mono- meric protein might not endow the enzyme with the property of a regulatory?enzyme. Calcium ion was found to affect the enzyme in two ways. One is to enhance the enzyme reaction rate by facilitating the binding of phosphate?group bearing substrate molecules with the enzyme, and the other is to reduce the activity of enzyme by enhancing the formation of protein aggregate. Since?the inhibitory effect of calcium ion can be partially reversed by dilution, calcium ion may play a part in the reversible association-dissociation pheno-?menon of the enzyme studied by the molecular sieve chromatographic technique. From the available data, the turnover number of enzyme toward 5'-AMP was calculated as 1.94 x 105 moles per mole enzyme (MW7 x 104) per minute.