We evaluated multidrug resistance (MDR) in human bladder cancer cell lines UM-UC-2, UM-UC-6, UM-UC-9 and the UM-UC-6dox subline induced to doxorubicin resistance by in vitro doxorubicin exposure. We compared the profile of multidrug resistance in these cell lines with that of the UM-UC-3 human renal cancer cell line. Of these cell lines, UM-UC-2 was most sensitive to both doxorubicin and etoposide, while UM-UC-6, UM-UC-9 and UM-UC-3 showed 1.5-, 2.1-, and 5.4-fold more resistance to doxorubicin than UM-UC-2 cells. These cell lines were also more resistant to etoposide than UM-UC-2. Addition of verapamil at 10 microM. reduced the doxorubicin resistance in UM-UC-6 and UM-UC-6dox cells, but UM-UC-9 cells showed little change in doxorubicin sensitivity in the presence of verapamil. In a model of intravesical (short-term) treatment verapamil increased the doxorubicin sensitivity of UM-UC-6dox but not that of UM-UC-6 cells. This effect in UM-UC-6dox cells was enhanced by continuously treating with verapamil after doxorubicin had been removed. Western blot analysis with rabbit anti-human P-glycoprotein polyclonal antibody demonstrated a distinct increase in P-glycoprotein in the resistant cell lines as compared with UM-UC-2. P-glycoprotein expression was roughly proportional to the degree of resistance to both doxorubicin and etoposide, but did not always correlate with the effect of verapamil on decreasing doxorubicin resistance. These results suggest that multidrug resistance is an important phenomenon in bladder cancer and that more than one pathway of multidrug resistance may be present in human bladder cancer cell lines.