A multidrug-resistant (MDR) subline of the immunoblastic B lymphoma cell line was established by sequentially selecting in increasing concentrations of adriamycin. The adriamycin-resistant cell line (HOB1/ADR) demonstrated resistance to a wide spectrum of chemotherapeutic agents including MDR drugs (Vinca alkaloids and anthracycline), antimicrotubule drug (colchicine), and DNA-damaging agents (cisplatin and mitomycin C). The expression of human mdr1 gene, as analyzed by RT-PCR and Western blotting, revealed a 13-15-fold increase in resistant cells. Unexpectedly, HOB1/ADR cells demonstrated a lack of reduced accumulation and of enhanced efflux of adriamycin. More than 60% adriamycin was effluxed at the same rate in both cell lines within 10 min. In contrast, the initial rate of vincristine accumulation was reduced by 3 fold in this resistant cell line. The maximal level of vincristine accumulation was 50% lower in the resistant cells than the parental cells. The maximal efflux rate was enhanced by 5 fold in the resistant cells. Inhibition of vincristine resistance by verapamil associated with restoration of drug accumulation, suggesting that acquired resistance in these cells is due to P-glycoprotein. These studies demonstrated that immunoblastic B lymphoma cells selected for adriamycin resistance preferentially developed P-glycoprotein-mediated vincristine efflux which plays an important role in vincristine resistance. In contrast, the resistant cells did not elevate adriamycin efflux, suggesting an additional mechanism responsible for adriamycin resistance.