The colony and cluster forming capacity of bone marrow and mononuclear leucocytes and colony:cluster ratio (CCIR) was studied in 23 patients with resistant anaemia and hypercellular bone marrow. Bone marrow controls included 28 patients with iron deficiency, haemolytic, megaloblastic and secondary anaemia as well as 15 haematologically normal subjects and 13 healthy volunteers. Colony formation by blood leucocytes was studied in aregenerative anaemias and in 13 healthy volun‐Colonies and clusters were grown in soft agar cultures in vitro from all control subjects. In normal volunteers the marrow grew between 24 and 201 colonies and 37 to 212 clusters/105 cells with the CCIR of 0.48–2.50. Blood leucocytes produced 6–184 colonies and 6–88 clusters/106 mononuclear cells with CCIR 0.43–2.64. Colony formation and CCIR were largely normal in haematologically intact subjects and patients with anaemia of known etiology. Patients with aregenerative anaemia showed abnormal colony formation by bone marrow and blood cells more often (P < 0.01) than control groups. Only 7 of 23 patients had colony formation within the normal range. Blood leucocytes in 12 out of 18 aregenerative anaemias failed to grow colonies in vitro. The marrow CCIR was below the normal range in all aregenerative anaemias with defective and in 3 with normal colony formation and was significantly lower than in the controls (P < 0.01). Of 19 aregenerative anaemias with defect colony formation 11 died, 7 with acute leukaemia. The in vitro colony formation appears useful for subclassification of anaemic conditions and for identification of patients who may be at higher risk of developing leukaemia.