A lectin from the seeds of Hura crepitans has been purified to homogeneity by affinity chromatography on acid-treated Sepharose CL-6B, followed by elution with D-galactose. The lectin is a glucosamine-containing glycoprotein with a molecular weight of 120 00, as determined by sucrose density gradient centrifugation, and consists of identical subunits with molecular weights of 30 000. The amino acid composition and total neutral sugar content are given. The Hura lectin agglutinates directly erythrocytes from several species, without specificity for human blood groups. In all cases, with the exception of pig erythrocytes, agglutination was enhanced by neuraminidase. Agglutination was inhibited, in decreasing order of potency, by N-acetyl-D-galactosamine, by D-galactose and galactose-containing oligosaccharides. The lectin has mitogenic activity for purified human T lymphocytes but not for B lymphocytes, and the activity is still evident at a concentration as low as 10 ng/ml. The specific mitogenic activity increases throughout the purification process.