On the basis of our unique idea that damage of the stromal tissue might result secondarily in damage of the cancer cells, an attempt was made to administer a fibroblast-inhibiting agent, chloroquine, in combination with mitomycin C in 20 patients with malignant tumors. 1) Of the 20 cases treated, the combined chemotherapy was effective in 6, induced no significant change in 6, caused aggravation in 3, and its evaluation was impossible in 5. 2) Of various malignant tumors treated, excellent therapeutic effects were obtained in patients with pleuritis carcinomatosa.. Among subjective complaints this regimen was most efficacious in alleviating pain. 3) Side effects of chloroquine were slight and its prolonged administration was possibl. Leucopenia often associated with administration of antitumor agents was relatively rare. 4) Accordingly, the combined chemotherapy with chloroquine and mitomycin C appears to be recommendable in the treatment of human tumors from the unique chemotherapeutic point of view.