Aspirates from bone marrow, spleen and liver were analyzed in 15 untreated patients with chronic myeloid leukaemia (CML). The proportion of erythroblasts was higher in the spleen and liver than in the bone marrow. An increased frequency of basophilic leucocytes was recorded in the extramedullary sites compared to the bone marrow. In the liver aspirates a considerable admixture of lymphocytes was found. The mitotic indices of the granulopoietic and erythropoietic precursor cells were lower in the spleen and liver than in the bone marrow. The results indicate that the composition and proliferative activity of the haemopoietic tissue differ in bone marrow and extramedullary sites in CML. The discrepancies may be due to differences in the microenvironment for the haemopoietic cells or to a selective accumulation of cells with a low mitotic activity in extramedullary sites. Since the final blastic transformation of CML may originate outside the bone marrow, further studies of the extramedullary haemopoietic tissue may be worthwhile.