ECENT CLINICAL OBSERVATIONS with oral antibiotics have demonstrated improved absorption and synthesis of erythropoietic factors and have awakened interest in the role that the bacterial flora of the gastrointestinal tract may play in the metabolism of vitamin B~2, folic acid, and aseorbic acid. A brief discussion of the factors influencing the bacterial flora of the gastrointestinal tract is necessary for proper interpretation of the pathologic status associated with impaired erythropoiesis. THE NORMAL BOWEL Numerous studies have been made to define the fecal flora. Unfortunately, such observations do not define the population in the small bowel, where absorption of nutrients take place. Postmortem observations are inaccurate because of the rapid contamination of the entire bowel with bacteria. Studies during life have been made by intubation with sterile tubing and by sampling the bowel lmnen during sm'gical procedures. The most accurate measurements of bacterial growth have been obtained by needle aspiration of the bowel at laparotomy for procedures not involving the stomach and small bowel. The stomach and small bowel, up to the lower ileum, are relatively sterile in children* and adults/ The organisms cultured were similar to oral flora and probably represent transient flora rather than any persistent bacterial growth. Even in the lower ileum 50 per cent of the aspirations were sterile. Studies by swabbing the open mueosal surface during surgical procedures have demonstrated similar results. It should be emphasized that such