An increased red cell adenosine deaminase (ADA) activity (85-fold) was found in a 10-year-old male child suffering from severe hemolytic disease. Evidence is given that the excessive ADA activity in erythrocytes is due to an abnormal amount of a catalytically and immunologically normal enzyme. Metabolic studies with the patient's erythrocytes show that the low ATP concentration in these cells (64% of comparably reticulocyte-rich blood) is due both to a diminished synthesis of adenylic nucleotides from adenosine, and to an excessive catabolism of AMP.