An adriamycin (ADM)-resistant variant (K562/ADM) of human myelogenous leukemia K562 was established. K562/ADM was stable for 2 months in medium without ADM, and was 130-fold more resistant to ADM as compared to the parent K562. Twenty clones were isolated from K562/ADM by the limiting dilution technique. Five clones with different ADM sensitivity were selected and characterized further. The extent of clonal resistance to ADM was parallel to the extent of resistance to vincristine (VCR), except for one clone, KA-15. The majority of clones, including K562/ADM, accumulated far smaller amounts of daunomycin (DAU) or VCR as compared to the parent K562. However, a highly resistant clone did not necessarily accumulate less DAU in the cells, indicating that the mechanism of ADM resistance cannot be explained solely by a defect of ADM accumulation. All clones rapidly transported DAU and VCR from the cells. K562/ADM expressed on the cell surface three distinct glycoproteins with molecular weights of 180,000, 83,000 and 65,000 daltons. No change was detected in the actin and tubulin contents of K562 and clones. K562/ADM and its clones expressed double minute chromosomes and contained homogeneously staining regions in the chromosomes.