Lacassagne, de Jongh, and Burrows and Kennaway have recorded the appearance of stratified, squamous, keratinizing epithelium in the prostates of mice which have undergone prolonged treatment with oestrin. Subsequent work by the author has shown that the organ characteristically affected in this way is the coagulating gland.1 This organ was commonly regarded as a part of the prostate until Walker in 1910 first discovered its separate nature and function. Further observations have now been made upon the effects produced by oestrin on the coagulating gland and prostate, and they appear worth consideration in connection with human pathology. Lesions have been brought about in the coagulating gland and prostate of the mouse by four closely allied oestrogenic substances, namely ketohydroxyoestrin, ketomethoxyoestrin, trihydroxyoestrin and equilin2 (Chart I). A fifth compound, equilenin, which is of somewhat similar structure, has