CAFC week types significantly differ in their drug resistance. The proporton of CAFC week types in PBPCs enriched by immunoaffinity removal of lineage-positive cells and further exposed to different drugs was evaluated by 5-week culture over irradiated M2-10B4 feeder (n = 7). This assay allows week-by-week enumeration of CAFC frequency and indicated that early-appearing (and possibly less quiescent) CAFCs were significantly less drug resistant than late CAFCs (P < .01 by ANOVA). Moreover, CAFC frequency was similarly enriched in PBPCs exposed to both mafosfamide and taxol and in PBPCs cultured for 7 days with 5-FU, SCF, and IL-3.