Abstract: The predominant effect of pH change on multidrug resistance (MDR) independent of active drug efflux has been demonstrated in tumor cells through studies of the drugs doxorubicin and daunomycin. Specifically, the distribution of both drugs was monitored in fibroblasts and myeloma cells to examine the role of pH in drug partitioning. The invention comprises in one aspect the treatment of MDR by administering a therapeutically effective amount of a pH modulator. Diagnostic utilities are contemplated and extend to drug discovery assays and methods for measuring monitoring the status of the onset or development of pH-related conditions such as MDR, as well as the measurement of intracellular drug accumulation. Therapeutic compositions include a composition comprising a pH modulator alone or in combination with the dose-limited therapeutic agent(s), and a pharmaceutically acceptable excipient, are also contemplated.