We report here a case of red cell adenylate kinase (AK) deficiency associated with hereditary hemolytic anemia. The proband is a 10-year-old Japanese girl. Her physical and mental development was normal. She has shown moderate to mild hemolytic anemia since the neonatal period and hepatosplenomegaly. The red cell AK activity was 44% of normal. Contents of red cell glycolytic intermediates and adenine nucleotides were normal when compared with a comparable reticulocyte-rich control. Glucose consumption and lactate formation were normal. Hexose monophosphate shunt activity was somewhat lower than that of a comparable reticulocyte-rich control. There were no significant differences in the contents of adenine nucleotides between the younger and older red cells of the patient. Enzymatic characterization by hemolysate revealed that the patient's AK had an increased Michaelis constant for adenosine diphosphate and slight thermal instability. The patient's enzyme migrated approximately half-way between the AK 1 and AK 2 position on starch-gel electrophoresis. The mode of inheritance of this case is obscure. The mechanism of hemolysis might be a structural gene mutation that caused altered electrophoretic and kinetic properties.