To investigate the relationship between oncogene activation and appearance of multidrug resistance (MDR) we transfected the human breast epithelial cell line MCF-10A, negative for the expression of the P-glycoprotein, with c-Ha-ras and/or c-erbB-2 oncogenes. The appearance of the MDR phenotype was then studied by evaluating mdr-1 mRNA expression, the presence of P-glycoprotein on the cell membrane and the onset of doxorubicin resistance, together with the effect of the reversing agent verapamil. We found that only MCF-10A transfected with both c-Ha-ras and c-erbB-2 oncogenes acquired the MDR phenotype.