An active, naturally-occurring panhemagglutinin has been described in the hemolymph of the protochordate Halocynthia pyriformis. This protein lectin is inactivated by temperatures exceeding 50° C, is active in the range pH 6-10, and is resistant to repeated freezing and thawing. Frozen hemolymph retains full hemagglutinating activity for several months. Activity is not reduced by dialysis against saline; however, serum proteins with total hemagglutinating activity can be precipitated by dialysis against deionized water or by treatment with appropriate ammonium sulfate concentrations. This lectin will not agglutinate human erythrocytes in the absence of Ca++; Ca++ potentiates the agglutination of red cells from all other species tested. However, Mg++ does not stimulate hemagglutination and will inhibit it at certain concentrations. Halocynthia pyriformis hemolymph will cause the agglutination of red cells from many avian and mammalian species. Human, rabbit, sheep, swine, goat, calf, ox, and pigeon erythrocytes agglutinate weakly with titers of 2-32; whereas, titers of 64-512 were consistently recorded for the red cells of guinea pig, horse, duck, goose, chicken, and turkey. The lectin was not specific for the ABO blood group antigens of man. The hemagglutinin shows considerable cross-reactivity; adsorption with nonhuman mammalian erythrocytes not only decreases activity toward the adsorbing cells but also alters the agglutination of cells from other species. Treatment of red cells with pronase or trypsin causes markedly increased hemagglutination titers; such treatment allows human erythrocytes to agglutinate in the absence of Ca++ in the medium. Incubation of hemolymph with N-acetylneuraminic acid strongly inhibits subsequent hemagglutinating activity, suggesting that sialic acid residues are present in the binding site of the lectin.