The introduction of irradiated ergosterol as a new source of vitamin has brought with it numerous problems for consideration. Animal experimentation has established the fact that irradiated ergosterol will prevent and cure rickets if it is given in sufficient quantity and that toxic effects are produced in massive doses. Controversy, however, has arisen in the application of these results to human beings, particularly in regard to prophylaxis and toxicity. Accordingly, Nov. 1, 1928, we undertook a clinical investigation of a biologically standardized preparation of irradiated ergosterol with the following problems in mind: (1) the determination of the amount of irradiated ergosterol necessary to prevent rickets in infants during the first year of life, when its administration was started in their first weeks; (2) the determination of the therapeutic dosage in rickets; (3) the possible development of toxic symptoms due to overdosage; (4) the possible advantages, if any, to be gained