Drug sensitivity of clonogenic leukemic blast cells was measured using the leukemic blast colony formation technique. Each of three chemotherapeutic agents, cytosine arabinoside, daunorubicin and aclarubicin, was added at the start of culture, and sensitivity was measured in terms of numbers of surviving leukemic blast colonies formed under continuous exposure to the drug. A substantial number of colonies was observed in all cases examined when the conditioned medium of human bladder carcinoma cell line 5637 was used as the leukemic blast colony stimulator. Although drug sensitivity varied considerably among the cases examined, sensitivity to administered drugs in clinical non-responders was significantly poorer than that in clinical responders (p less than 0.05).