Colonies derived from leukemic blast progenitors were sequentially observed on Days 3, 5, 7, 10 and 14 to evaluate the relation between the kinetics of leukemic blast progenitors and types of acute nonlymphocytic leukemia (ANLL), chemotherapeutic response and leukemic cell kill kinetics. The cells were obtained from 34 patients with typical ANLL and 6 with hypoplastic ANLL. The leukemic colony assay was performed. Colony-counts (CCs) were somewhat higher in M2 and M5 patients than in patients with other FAB subtypes. CCs were significantly lower in patients with hypoplastic ANLL than in those with typical ANLL (22±25 vs 148±205, P<0.02). Time required for 50% reduction in CCs from maximum CCs (Days for 50% reduction) was longer in relapsed than in previously untreated ANLL patients (6.5±2.3 days vs 3.3±1.5 days, P<0.01). In patients with remission both of CCs on Day 7 and maximum CCs were higher than in patients without remission (198±187 vs 69±77, P<0.05 for CCs on Day 7/233±188 vs 107±66, P<0.05 for maximum CCs). Time for 50% reduction was significantly shorter in patients with remission (2.4±0.7 days vs 4.2±1.2 days, P<0.02). The correlation between 「days for 50% reduction」and「acceleration of decrease at PhaseⅡ of leukemic cell kill curve」was noted. These findings indicate that the sequential observation of colonies derived from leukemic progenitors is useful not only for the prediction of the chemotherapeutic response but also for analysis of pathophysiological aspects of ANLL.