A method of contact microradiography has been adopted for the measurement of the amount of epidermal protein produced by epidermal cells during differentiation. A new x-ray tube suitable for an analysis at cytological level has been devised. There is a progressive increase in the amount of x-ray absorbing substance which is connected with the aging of spinous cells as they ascend from the lower to the higher levels of the malpighian layer. Although wide variations occur from specimen to specimen, a synchronism is nonetheless apparent in the rate of accumulation of these substances in the cell population of any given specimen. Such a synchronism was lacking in specimens of neoplastic epidermis and in pemphigus.