A mitogenic lectin for human lymphocytes is present in the marine sponge Desmapsama anchorata. The protein hemagglutinates red blood cells irrespective of ABO group antigens. We now report the isolation of this lectin, by affinity chromatography on a column of raffinose conjugated to epoxy-activated Sepharose 6B, in 8.3% yield and with a purification index of 27 based on hemagglutinating activity. The isolated lectin is a glycoprotein with two subunits with molecular weights of about 18 and 36 kDa which display carbohydrate combining sites of similar specificities and can be associated in different forms.