1. The response to cyanocobalamin and folic acid therapy was studied in forty-seven subjects with severe nutritional megaloblastic anaemia; twenty-five were vitamin B12deficient and, of these, ten were also folic acid deficient.2. In twenty-four of the subjects, absorption of orally administered cyanocobalamin was normal, and there was a haematological response to small doses of orally administered cyanocobalamin, indicating adequate absorption. In one subject there was impaired cyanocobalamin absorption although free gastric hydrochloric acid was present.3. The findings indicate that the absorptive function of the small intestine was unimpaired, with one exception, although there was a possible defect in nucleoprotein synthesis associated with a lack of vitamin B12and folic acid. Addisonian pernicious anaemia was not found in any of the subjects.