Doctors Lansing, Alex, and Rosenthal propose that in human arteriosclerosis the formation of intimal plaques is conditioned to a large extent by degeneration and calcification of elastic fibers in the media of the artery. In support of this concept they are reviewing earlier data from their laboratory and are presenting new pertinent experimental observations. The substance of this paper was delivered at the second annual scientific meeting of The Gerontological Society held in Chicago, November 5-6, 1949.