A. hypogaea hemagglutinin was purified by ammonium sulfate fractionation and Sepharose 6 B column chromatography. The homogeneity of the purified hemagglutinin was ascertained by ultracentrifugal analysis and polyacrylamide gel electrophoresis. It has a molecular weight of 106.500 and is a tetramer of a subunit with a molecular weight of 27.000. The purified hemagglutinin agglutinated neuraminidase-treated human erythrocytes regardless of their ABO group type, but did not agglutinate intact erythrocytes. In hapten inhibition assays with simple sugars, the so-called Mäkelä's group 2 sugars, which bear the same configuration of hydroxy groups at C-3 and C-4 as D-galactopyranose, were inhibitors for this hemagglutinin. It does not contain any carbohydrate, in contrast to most phytohemagglutinins except concanavalin A and wheat germ agglutinin.