The C-nucleoside antibiotic, showdomycin, was found to be a low affinity but irreversible inhibitor of the nucleoside transport system of both the murine L1210 leukemia and the murine bone marrow progenitor cells in vitro. The antibiotic was twice as toxic to L1210 cells as to bone marrow progenitor cells, and this differential was further increased several fold in the presence of cytidine. Cytidine also reduced the toxicity of showdomycine in vivo when administered with the antibiotic, and afforded an increased life span of L1210 bearing mice of from 30 to 40 percent.