1. The vitamin C in the dietary of an institution was largely destroyed by the methods of cooking and distribution.2. Some 50 mg. of ascorbic acid per head per day were required to be added to the diet to produce an optimum excretion level.3. Large doses of ascorbic acid were given to a group of adolescents in the institution over a period of several months. A record was kept of the incidences of infectious diseases in this treated group and in the remainder (controls). The following conclusions were reached:(a) The incidences of common cold and tonsillitis were the same in the two groups.(b) The average duration of illness due to the common cold was the same in the two groups.(c) The duration of illness of tonsillitis was longer in the control group than in the test group.(d) Cases of rheumatic fever and pneumonia occurred in the control group but no case of either disease occurred in the test group.