Although plant hemagglutinins, which have been known nearly as long as specific immune agglutinins, were long thought to be nonspecific, some have been found to have a blood-group specificity which is quite sharp. Some of these agglutinins, such as the lectins of Dolichos biflorus and Ulex europeus, have found important practical applications in blood group techniques. Other group specificities have been found, and even some of the plant extracts which agglutinate all human erythrocytes probably have a measure of specificity, since they are specificially inhibited by certain carbohydrates. The information that may be gained in this way concerning reactive structures in the red cell is discussed. A study of such reactions led Mäkelä to classify all aldopyranoses into four groups, depending on their configuration at carbon atoms 3 and 4. The inhibition reactions of human anti-Rh sera suggest that this classification may be of significance in this connection also.