SUMMARYBlood group specific plant agglutinins (named lectins by the present author) were discovered in 1945, first mentioned in 1947, and described in detail by Renkonen in 1948 and by Boyd in 1949. Similar agglutinins have since been found in some invertebrates. Thousands of species have now been screened for such activity. The commonest activities are anti‐A, anti‐H, anti‐N, and someimes anti‐B and anti‐M. A specificity apparently new was found in Arachis hypogaea, and a report of a specificity pertaining to the Rh‐system has appeared. Lectins have been used in routine blood grouping, in the determination of the subgroups of A and AB, in the diagnosis of secretors, and in various theoretical investigations. They have been used to estimate the minimum number of moleules per red cell required for agglutination and in attempts to determine the number of molecules that can combine with a red cell. Lectins have been purified and characterized. Their quantitative precipitin behavior with blood‐group subtances has been studied. It has been possible to alter the specificity of lectins by selective inhibition and by chemical modification.