In addition to the established efficacy of antibiotics in the prevention and treatment of diseases due to infectious agents, there are numerous observations which suggest that they may influence the nutrition of animals and man. Some reports indicate that certain antibiotics enhance the growth of several animal species and also of infants and children,1and although the mechanisms of this action are not clearly understood, the observed effects frequently have been attributed to an alteration of the bacterial flora of the gastrointestinal tract. Such alterations might influence nutrition by increasing or decreasing the quantities of certain nutrients available to the host through synthesis or utilization of vitamins and possibly of other essential nutrients by the bacteria in the intestinal tract. The possibility that certain antibiotics may influence nutrition by some direct effect on tissue metabolism must also be considered. Physical changes resembling those which occur in patients with vitamin