Using colony assays for human erythropoietic (BFU-E, CFU-E) and granulopoietic (CFU-C) progenitors, normal and leukemic myelopoietic differentiation were compared; similar patterns were found in both. However, the origin of blast cells characteristic of the disease could not be established indicating the need for a direct approach to these cells. A colony assay for blast cells in acute myeloblastic leukemia is described. Blast cell colony-formation is significantly correlated with blast cell number, and the colonies contain cells of blast like morphology without differentiation markers. It is proposed that this method, taken in conjunction with results from assays of myelopoiesis and lymphopoiesis, may provide a more complete picture of leukemic differentiation. It is anticipated that such a model will be useful in devising new therapies.