The nutritional value of processed soy protein (isolated soy proteins and soy-protein concentrates) in human protein and amino acid nutrition is evaluated on the basis of a review of studies of growth and nitrogen balance in infants, children, adolescents, and adults. Findings show that well-processed soy-protein isolates and soy-protein concentrates can serve as the major, or even sole, source of protein intake and that their protein value is essentially equivalent to that of food proteins of animal origin. The importance of the sulfur amino acid content of soy protein for practical human nutrition is also examined from nitrogen-balance data. Under conditions of an anticipated normal usage of soy protein, methionine supplementation is not only unnecessary but may even be undesirable for young children and adults. However, for newborns, the data suggest that modest supplementation of soy-based formulas with methionine may be beneficial. Soy proteins have also been found to be of good quality to include in hypocaloric diets for weight reduction in obese subjects. Finally, the data indicate that soy proteins are well-tolerated and of good acceptability.