We describe here investigations into the ability of a tiapamil derivative, Ro44-5912 to overcome multidrug resistance (MDR) in doxorubicin (ADR)-resistant murine leukemic P388 cells. This compound has the formula: C27H39NO4S2.1:2C6H8O6, a M. W. of 858 and is structurally similar to verapamil, an established inhibitor of P-glycoprotein (PGP). We have compared the MDR modulating properties of Ro44-5912 with verapamil in P388ADR cells. Doxorubicin concentration required to achieve 50% inhibition of growth (IC50) for P388ADR cells was found to be 24 microM. In contrast, treatment of P388ADR cells with Doxorubicin and 3 microM verapamil decreased the IC50 value to 2.5 microM. A further decrease was observed with 3 microM Ro44-5912 treatment, where an IC50 value of 1.1 microM was obtained. Doxorubicin accumulation was also determined by flow cytometry in order to determine whether the increased levels of chemosensitivity observed for Ro44-5912 were reflected by increased cellular drug uptake. The results revealed that Ro44-5912, at equivalent concentration, increased doxorubicin accumulation in P388ADR cells beyond that obtained with verapamil whereas no effects were seen with the parental P388 cells. The effect of Ro44-5912 on the binding of C219 monoclonal antibody to PGP in MDR cells was also studied and found not to decrease C219 expression on P388ADR cells.