Plucking feathers leads to a slight temporary increase (2-7 hrs.) in rate of cell division in the underlying epithelium of the crop. A further slight and temporary increase in mitotic rate is produced by splitting the skin or by intradermal inis. of 0.5 ml. of control substances such as air. water and 0.9% NaCl. This temporarily increased mitotic rate is consonant with successful use of the method, but pronounced wounding of the overlying skin and also the presence of irritants in the material injected induce a prolonged increase (24-48 hrs.) of mitotic rate together with a positive visual response. Many substances tested at 48 hrs. after their first inj. are apparent irritants since they produce a positive visual response together with an increased mitotic count. It is obvious that the material used for routine tests for prolactin must not contain any of these substances in the quantities indicated. Negative results were obtained with estrone. 0.5 mg. and stilbestrol. 4.5 mg. Doses of 1-2 mg. of the wound substance, leukotaxine. produced prompt increase in the rate of mitosis which was observed at 2-24 hrs. but not at 48 hrs. thereafter; slight or questionably positive visual responses were observed at 24 and 48 hrs. after inj., and mitosis was not increased after 48 hrs. The great sensitivity of the micro-test for prolactin probably depends in part upon the fact that incidental wound effects fall only slightly below a positive response.