Chloroquine diphosphate, as demonstrated to be a fibroblast-inhibiting agent, was employed in an attempt to suppress the growth of cancer indirectly by inhibiting the interstitial connective tissue of cancer. Bashford cancer which is relatively rich in the interstitium was tested with this agent and the following results were obtained. 1) Chloroquine diphosphate clearly retarded the growth of Bashford cancer. The daily intraperitoneal dose of 25mg/kg was more effective than 10mg/kg. 2) The life span of the tumor-bearing animals was slightly prolonged as compared to controls. 3) The above data were supported by the collateral experimental results; an increase of tumor tissue necrosis as shown by histological and histochemical studies, a decrease of connective tissue elements and acid mucopolysaccharide, inhibition of lowering of liver catalase activity, and a decrease of iron in the tumor and other organs.