Inquiries carried out at this Institute, during the past few years, into the causation of cancer by specific chemical compounds have led to a search for some substance which might occur in the living body and be the cause of malignant growth. It had been found (Cook and Dodds, Cook, Dodds and Hewett) that certain of the carcinogenic compounds synthesised by Cook and his collaborators possessed oestrogenic properties, also. Such an observation at once suggested a series of experiments on the effects of oestrin when applied to mice. Although some of the results of subcutaneous injection of oestrin have been described already by Lacassagne and other workers, no comprehensive paper on the matter has yet appeared. Moreover, the subject has derived a fresh interest, in connection with cancer, from the work of Cook, Dodds and Hewett alluded to above. While studying this subject we have made two observations at least which appear to be novel, namely (1) that definite changes may be brought about in the urogenital organs by applying oestrin to the non-epilated skin of the back, and (2) that scrotal hernia frequently follows the application of this hormone to male mice. A paper on the latter subject is now awaiting publication and it will be unnecessary to mention the matter here otherwise than by reference in passing. The following remarks concern the effects on mice when oestrin is painted upon the non-epilated skin. The preparation used was ketohydroxyoestrin for which we are indebted to the generosity of Dr. Girard. This sample, prepared from the urine of pregnant mares, has a specific rotation [α]D = + 178°.