The chemotherapeutic drug sensitivity of live cells freshly isolated from two human bladder tumors was studied. Substantial differences in the sensitivities of the two tumors to various chemotherapeutic drugs as well as different sensitivities of a given tumor to different drugs were found. Such data could be valuable clinically. The therapeutic value of cyclophosphamide, thiotepa and methotrexate was questionable for these two tumors because an increase in thymidine incorporation was seen after treatment with these drugs. A similar increase was observed after treatment with 5-fluorouracil in cells of a tissue-culture cell line derived from a human bladder tumor. Pretreatment of the cells with 5-fluorouracil reduced this effect. Even after 12 to 18 hours of exposure to 5-fluorouracil, the cells formed a confluent monolayer within 9 days. A similar treatment with doxorubicin hydrochloride resulted in detachment of the cells and formation of aggregates. The aggregated cells excluded trypan blue for at least 9 days after the doxorubicin hydrochloride was removed.