1. AMP-deaminase activity in erythrocytes increases gradually during chick (Gallus domesticus) maturation, reaching the adult level of enzymatic activity at about 16 weeks after hatching. 2. Adenosine deaminase activity increases approximately two-fold during this period. 3. Substrate specificity and immunoinhibition studies indicate that erythrocytes from adult chickens and newly-hatched chicks contain the same AMP-deaminase isozyme. 4. Comparison of temporal changes in RBC AMP-deaminase with those previously described for this enzyme in muscle and brain suggests that the level of this enzyme is regulated differently in these tissues.