To determine whether the biological characteristics of leukemic cells change after repeated chemotherapy, we compared the proliferative activity and drug sensitivity of leukemic blast progenitors in 7 patients with acute myeloblastic leukemia at diagnosis and in relapse. The proliferative activity of leukemic blast progenitors was assessed based on primary (PE1) and secondary (PE2) colony formation in methylcellulose culture and on the recovery of clonogenic cells in suspension culture. The effect of cytosine arabinoside (Ara-C) on leukemic blast progenitors was studied both in methylcellulose and in suspension cultures. PE1 and PE2 values varied among the patients. PE2 of 4 patients out of 7 patients became significantly higher in relapse than at diagnosis. The sensitivity to Ara-C of leukemic blast progenitors deteriorated in 5 patients in relapse. The results suggest that the biological nature in terms of proliferative activity and Ara-C sensitivity of leukemic blast progenitors may change in the clinical course after chemotherapy.