Phenylalanine methylester (PME), a lysosomotropic compound can be used to deplete monocytes and myeloid cells from peripheral blood and bone marrow (BM). The potential of PME for purging leukemic cells from BM was investigated using U937 and HL-60 cell lines as models. Optimal purging conditions for U937 cells were determined using an MTT assay (3-4, 5-dimethylthiazol-2, 5-diphenyl tetrazolium biomide; Sigma). Elimination of U937 cells was time-, temperature-, and dose-dependent. PME activity was optimal at 37 degrees C for 45 minutes. Depletion of U937 was > 2.8 logs for 50 mmol/L PME. Compared with another purging agent, 100 micrograms/mL 4-hydroperoxycyclophosphamide had activity comparable to 40 mmol/L PME. HL-60 cells were even more sensitive to PME than U937 cells. To support observations made with the MTT assay, clonogenic assays were performed. PME, 50 mmol/L at 37 degrees C resulted in total depletion (> 5 logs) of U937 colonies. Progressive depletion of normal progenitor cells occurred when BM was incubated with PME at concentrations from 5 to 100 mmol/L. At 37 degrees C, 50 mmol/L PME reduced colony-forming units-granulocyte-macrophage and burst-forming units-erythroid (BFU-E) recovery by 98%. Recombinant human mast cell factor augmented BFU-E after PME treatment but had no effect on HL-60 or U937. These studies suggest that PME deserves further study as an agent for ex vivo marrow purging.