AMP deaminase [AMP aminohydrolase, EC 3. 5. 4. 6] from chicken erythrocytes was purified using ammonium sulfate fractionation, and DEAE-cellulose and phosphocellulose chromatographies and the effects of some nucleotides and alkali metal ions on the enzyme were studied. The specific activity of the purified preparation was 140 μmoles of AMP deaminated/min/mg of protein. The enzyme showed an optimal pH at 7.1 in both presence and absence of activators. ATP, ADP, GTP, and alkali metal ions, which are allosteric activators, protected the enzyme against heat inactivation. The plot of reaction rate against AMP concentration was sigmoidal in the absence of any ligands, indicating that there was a homotropic interaction between the enzyme and AMP. However, in the presence of ATP or alkali metal ions, the plot became hyperbolic, suggesting that the homotropic interaction with AMP molecules decreased. The enzyme also showed a cooperative interaction with respect to ATP. The maximal velocity of the reaction was the same in the presence and the absence of activators. Kinetic analysis of the cooperative interactions of the enzyme with AMP and ATP suggested that there were two binding sites for these ligands.