The following cytostatic agents were tested for activity in vivo and in vitro inWalker carcinosarcoma 256 of the rat: cyclophosphamide, triaziquon, 5-fluorouracil, methotrexate, adriamycin, dactinomycin, daunorubicin, hydroxyurea, procarbazin and vincritine. With the exception of vincristine, the results of therapy in vivo could be predicted by using a rapid in vitro test system. This involved, for cyclosphosphamide, triaziquon, adriamycin, and daunorubicin, the measurement of 3H-uridine or 3H-thymidine incorporation. The activities of methotrexate and 5-fluorouracil could be determined from 3H-deoxyuridine incorporation and that of dactinomycin from 3H-uridine incorporation. The results of short-term tests (uring adriamycin, daunorubicin, and dactinomycin) in roughly 100 human tumors were compared with data in the literature on therapy with the same cytostatic agents. Good agreement was found between the results of in vitro tests and the literature data on clinical therapy.