Grafts of a transplantable mouse sarcoma designated as No. 180, and those of an attenuated strain of a more malignant Sarcoma S/37, treated with testicle extract, either fail to grow on inoculation or result in tumors of a lower growth rate than the controls. Autografts of spontaneous mouse tumors so treated show little if any effect, while the Bashford adenocarcinoma and the unattenuated S/37 are unaffected. The factor in testicle extract responsible for the retarding activity passes readily through a Berkefeld filter and is thermostable.