During the course of prolonged high estrogen treatment the glandular interstitial tissue of the testis in mice of the Strong A strain has been observed to hypertrophy, in a few instances to such an extent that large areas of the testis were composed entirely of these cells.1Since neither local invasions nor metastases were observed, not one of these hypertrophies or overgrowths was considered malignant. Recently, however, a malignant tumor of the interstitial cells appeared in an A strain mouse receiving estradiol benzoate. This mouse had received weekly subcutaneous injections of 0.05 mg. of estradiol benzoate2in sesame oil beginning at the age of 30 days. Shortly after six months of treatment the left testis began to increase in size rather rapidly, and after eight months it was several times normal size, was nodular and protruded somewhat above the base of the tail. Shortly thereafter the general health