Although very high doses of 5-fluorouracil was used in the weekly 24-h infusion, high-dose 5-fluorouracil (2600 mg/m2/week) and leucovorin (500 mg/m2/week) protocol, myelosuppression was surprisingly low. The current study was conducted to investigate the possible mechanism underlying the low myelosuppression. To mimic the clinical situation, peripheral blood progenitor cells collected from 12 patients were used for colony forming unit-granulocyte and monocyte clonogenic assay; and 2 representative modes of 5-fluorouracil exposure (30 min. versus 24 hr) were examined for cytotoxic effects on human myeloid progenitor cells. Previous pharmacokinetic studies have estimated the concentrations of 5-fluorouracil in the bone marrow to be 200-400 microM and 1-2 microM for the 30 min. infusion (600-900 mg/m2) and the 24 hr-infusion (1000-2000 mg/m2) regimens, respectively. The results of our colony-forming unit-granulocyte and monocyte clonogenic assay showed that 24-hr exposure to 5-fluorouracil (2 microM) and 30 min. exposure to 5-fluorouracil (100 microM) resulted in 27.2% and 78.2% inhibition of the colony formation, respectively. Our data provided direct evidence which may explain why myelotoxicity is significantly less in weekly 24 hr infusion of fluorouracil than in the conventional bolus regimens.