Alterations in the C-9 side chain of the anthracycline antibiotics, adriamycin and daunorubicin, have a profound effect on antibiotic uptake and accumulation by cultured L1210 mouse leukemia cells. The degree of inhibition of DNA and RNA biosynthesis in the L1210 cells is directly related to the cellular uptake and accumulation of the drug analogs. Polar drug metabolites, daunorubicinol and adriamycinol, retain inhibitory activity against nuclei acid metabolism, but have decreased membrane binding and permeability. Cellular uptake and accumulation of the C-9 analogs are inversely related to drug polarity. The polarity of the anthracycline analogs contributes to the differences in therapeutic index and in vivo activity through fundamental effects on membrane permeability, metabolism and macromolecular binding.