Substances which agglutinate red blood cells are widely distributed in nature, particularly in the seeds of leguminous plants. Some of these have been isolated in a high state of purity and carefully studied with respect to their chemical and physical properties, immunological behavior, and toxicological effects on animals. Of these, the hemagglutinins of the castor bean, jack bean, soybean, and varieties of Phaseolus vulgaris have received the most attention. The possible nutritional significance of those hemagglutinins which are found in edible legumes is discussed. A mechanism is proposed to explain the nature of the interaction of hemagglutinins with red blood cells, and their possible function in the plant.