In summary, the ingestion by adult men of an all-plant diet supplying 46 gm. protein per day, primarily as bread made from white wheat four, over a period of 74 days, resulted in positive balances for copper, iron, molybdenum, selenium, and zinc. Iron balances were significantly decreased by the isonitrogenous substitution of pinto beans or peanut butter for 20 percent of the nitrogen supplied by white bread. When pinto beans or white rice substituted for 20 percent of the nitrogen of white bread, the retention of molybdenum was lower and the retention of selenium was greater. Although the men were nitrogen balance and total plasma protein concentrations were normal, the ingestion of diets providing 46 gm. protein, from all-plant sources resulted in significantly lower plasma albumin, with corresponding elevations in alpha, beta and gamma globulins. However, the substitution of pinto beans for a portion of the white bread improved plasma albumin and alpha-globulin levels. Routine clinical data obtained on patients who previously consumed strictly vegetarian diets should include determinations of plasma albumin. The ability of the subjects to maintain nitrogen balance on protein intakes of 46 gm. per day from all-plant sources appeared to be correlated with decreases in the urinary excretion and plasma concentration of alpha-aminobutyric acid, a metabolite of methionine and threonine, suggesting conservation of methionine or re-utilization of this metabolite. The possible mechanism of this adaptation to minimal protein intake, or dietary protein from all-plant sources, is discussed.