Vitamin D5, a member of the family Vitamin D wich was discovered by Edward Mellanby in 1919 during his classic experiments with rickets, is about 180-fold less active than vitamin D3, in calcification of rachitic cartilage and about lOO- to 200-fold less active in induction of bonecalcium mobilization. In stimulation of intestinal-calcium transport, vitamin D5 is about 80 fold less active than vitamin D3.