DURING the past fifteen years there has been a continuing conflict of data and opinions regarding the comparative effectiveness of vitamin D from ergosterol, the plant sterol activated by irradiation with ultraviolet rays and marketed under the name of viosterol, and the vitamin D preparations from animal sources such as tuna liver oil and cod liver oil. The latter are dependent chiefly on activated 7-dehydrocholesterol for their antirachitic effectiveness, the greater proportion of this probably being present in cod liver oil. Previous studies have chiefly employed one of two methods. Either they dealt with groups of healthy infants or with groups of rachitic infants who were given the various forms of vitamin D through the winter period and for whom consecutive roentgenograms were made and the diagnoses of each group compared with that of the other corresponding group, or serologic studies of calcium and phosphorus of similar healthy and rachitic