A lectin present in the broad bean, Vicia faba, was isolated by affinity chromatography on Sephadex G-150. The active lectin had a molecular weight of 50 000. The lectin contained about 3% neutral sugar and it contained two different subunits. The subunits had molecular weights of 17 300 and 14 300. The lectin was mitogenic to human peripheral lymphocytes with some reduction in activity upon acetylation. The lectin competed with pea lectin for binding to murine tumor cells. Binding to tumor cells was inhibited by methyl-alpha-D-mannoside but not by D-galactose; binding was not influenced by the presence or absence of Ca2+, Mn2+ or ethylenediaminetetraacetate.