In a previous paper Hunter and Pinkus (1) reported that the oral administration of 150,000 units of vitamin A daily for one month decreased epidermal keratin cell counts in all of ten healthy volunteers. The question of the biologic mechanism that brought about this decrease is the subject of the present paper. MATERIAL AND METHODS Ten healthy adults (7 male and 3 female) ranging in age from 25 to 40 years volunteered for the study. The keratin layer was stripped off an area approximately 6 X

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... 25 em on the flexor surface of one forearm by repeated application of Scotch Tape® until a dry glistening surface was obtained. The stripped-off material was saved for counting horny cells as reported (1) . Under local xylocaine anesthesia, 2 mm punch biopsy specimens were removed from the stripped area 24, 48, 72, and 96 hours later (Series 1), and a control specimen was taken from normal skin at least 2 em distant to avoid spill-over of mitotic activation (2). The subjects then took one buccal doset of vitamin A, 150,000 units daily for 30 days. A similar area was stripped on the other forearm at the end of this period, and control and 24, 48, 72 and 96 hour biopsy specimens were taken (Series 2). A final strip and third series of specimens were done at the end of another month during which no vitamin A was taken. The specimens were fixed immediately in formol-alcohol and embedded in paraffin. Cells were counted in serial sections cut at 8jz and counts corrected according to Abererombie's formula as described by Pinkus (3). The number of resting and dividing nuclei was determined for one square millimeter. The number of horny cells per square centimeter was taken from published data (1) and divided by 100. RESULTS The experiment had two parts and two purposes.