The role of thymidine and hypoxanthine uptake in TMTX-resistance of murine myeloid progenitors in vitro. Unseparated murine BM cells were cultured in suspension dishes in the presence of cytokines and either undialyzed or dialyzed FBS. Some cultures also contained 10 μmol/L thymidine (Thd), 100 μmol/L hypoxanthine (Hypo), and/or 150 nmol/L TMTX. After 4 days of culture, equal volumes of cells were washed and plated in TMTX-free semisolid cultures to determine the myeloid progenitor content. Progenitor survival was calculated by dividing the number of CFU-Cs per volume from cultures containing TMTX by the number per volume from drug-free cultures, then multiplying by 100. The designation TP indicates that the serum was treated with thymidine phosphorylase. Each bar represents the mean of at least three independent experiments and the line shows ±1 SD from the mean.