The kinetic and molecular properties of AMP deaminase [AMP aminohydrolase, EC 3.5.4.6] purified from baker's yeast (Saccharomyces cerevisiae) were investigated. The enzyme was activated by ATP and dATP, but inhibited by P1 and GTP in an allosteric manner. Alkali metal ions and alkaline earth metal ions activated the enzyme to various extents. Kinetic negative cooperativity was observed in the binding of nucleoside triphosphates. Kinetic analysis showed that the number of interaction sites for AMP (substrate) and P1 (inhibitor) is two each per enzyme molecule. The molecular weight of the native enzyme was estimated to be 360,000 by sedimentation equilibrium studies. On polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate, the enzyme gave a single polypeptide band with a molecular weight of 83,000, suggesting that the native enzyme has a tetrameric structure. Baker's yeast AMP deaminase was concluded to consist of two “protomer” units which each consist of two polypeptide chains with identical molecular weight.