Induction of P-glycoprotein-related multi-drug-resistance (MDR) has been shown innormal and malignant tissues to result from environmental stresses such as heat shock, exposure to carcinogens or X-ray irradiation. To identify conditions under which MDR is enhanced during anti-neoplastic chemotherapy, a cell line showing low-level intrinsic MDR was investigated. In the pleural mesothelioma cell line, PXF1118, < 1% of cells ex- pressed P-glycoprotein (P-gp), as shown by immunocytochemical staining with monoclonal antibody (MAb) MRK16. Exposure of PXF1118 to vincristine, vindesine, vinblastine or doxorubicin for 2-3 weeks led to an increase in the MDR cell fraction of up to 15-28% during 2 to 3 weeks. For doxorubicin and vindesine, dose-dependence was observed: drug concentrations not capable of eliciting cytotoxicity failed to induce significant P-gp expression. Nutrient starvation in aging medium, exposure to activated cyclophosphamide (even at high concentrations) or cisplatin caused only negligible MDR induction. After exposure to vindesine for 6 weeks, tumor colonies exhibited highly enhanced resistance to Vinca alkaloids, doxorubicin, etoposide and dacarbacine, whereas their sensitivity to mitomycin, activated cyclophos hamide or cisplatin remained unchanged. As determined by [3H]-thymidine uptake and proliferation antigen expression, induction of MDR phenotype was observed at minimal proliferative activity with no change in cell count during exposure to anti-cancer drugs, thus suggesting that the drug treatments changed the phenotype of the cells rather than selecting for a resistant sub-population. In addition, changes in cell differentiation were observed during MDR induction. Induction of P-gp during exposure to anti-cancer drugs thus provides a model for MDR development during initially successful chemotherapy.