The platelet peroxidase (PPO) content of circulating thrombocytes was determined in 10 healthy controls and in 18 cases of acquired refractory anaemia (2 with refractory anaemia, 4 with sideroblastic refractory anaemia, 8 with refractory anaemia with excess of blasts, 3 with refractory anaemia in transformation and 1 with chronic myelomonocytic leukaemia). The thrombocytes of the controls were invariably PPO-positive. No peroxidase deficiency was found in the 4 patients with sideroblastic refractory anaemia. Of the remaining 14 cases PPO-positive and PPO-negative thrombocytes coexisted in 8. Only in 1 case of refractory anaemia with excess of blasts were circulating platelet peroxidase-positive micromegakaryocytes demonstrated. PPO deficiency seems to be an important dysthrombopoietic feature detectable only at an ultrastructural level.