A fibrob'ast-inhibiting agent, chloropuine, was employed to be treated against experimental animal tumors, Brown-Pearce cancer and Yoshida sarcoma. The results are as follows 1) A higher survival rate of the rabbits bearing Brown-Pearce cancer and inhibition of the growth of the tumor were observed by daily administration of chloroquine diphosphate of 10 mg/kg. However no significant results were obtained in the animals with far advanced cancerous involvement. 2) No significant life prolongation was obseved in the rats bearing Yoshida sarcoma by chloroquine administration of daily dosis of 25 mg/kg, but this ttreatment appeared to be inhibited accumulation of the ascites and increase of the ascites tumor cells. 3) As shown above, a fibroblast-inhibiting agent, chloroquine, used in the treatment of the animal tumors, led to reasonably good result.