The mechanism of red cell adenosine deaminase (ADA) accumulation in a case of hereditary haemolytic anaemia due to increased red cell ADA activity was investigated. ADA activity of the younger cells was twice that of the older cells. Rate of ADA synthesis in erythroid colony cells cultured from the patient's bone marrow cells was 11-fold greater than that from the normal. The accumulation of ADA in the patient seems to be due to the increased synthesis in precursors of red cells in spite of the increased degradation in peripheral blood.