Severe protein malnutrition is associated with a functionally and histologically definable malabsorption syndrome, mild to moderate in severity, which returns to normal with protein repletion alone i.e. without folic acid or vitamin B12 supplementation or antibiotic therapy. This condition presents itself in a clinical setting similar to, but not identical with classical tropical sprue. The objectives of the proposed research are: First, to determine which of the possible factors associated with protein malnutrition play a causative role in this syndrome; a) abnormal bacterial colonization of the small intestine, b) concomitant folic acid deficiency, or c) protein malnutrition per se. Second, to define both by light and electron microscopy, the natural history of the intestinal mucosal abnormalities observed in this syndrome during the malnourished state and at appropriate intervals during protein realimentation. After comprehensive studies in the malnourished state, patients will be observed for prolonged periods under therapeutic regimes designed to differentiate the separate effect of a-a high protein diet alone, b-the effect of therapeutic doses of folic acid, and c-the effect of eradication of an abnormal intestinal bacterial flora by antibiotic therapy. These observations will be correlated with similar studies in patients with classical tropical sprue.