Separation of immunostimulating polysaccharide (ATP) from Angelica and its biological activities were investigated. AIP was separated as an acetone-insoluble and non-dialysable fraction from hot water extract obtained by heating the root of Angelica acutiloba in water at 95 degrees-98 degrees for 30 min. It is a water-soluble heteropolymer(s) consisting of uronic acid, hexose and peptide. The anti-tumour activity of AIP was observed in terms of prolongation of the survival period of mice bearing Ehrlich ascites cells. The uptake of tritiated thymidine into murine and human spleen cells could be stimulated by AIP in a dose-dependent manner. Murine B cells were activated polyclonally by AIP and differentiated to anti-body-forming cells even in the absence of either helper T cells or macrophages. The possibility that the biological activity of AIP might be due to contamination by bacterial lipopolysaccharides (LPS) or lipid A-associated protein (LAP) was ruled out for the following reasons: (i) polyclonal B-cell activation by AIP was shown in spleen cell cultures of C3H/HeJ mice, a low responder strain to LPS; (ii) the activity of AIP disappeared completely after a mild periodate oxidation whereas that of LPS containing LAP was not lost by similar treatment. In addition, the primary antibody response to sheep-erythrocytes was markedly augmented by an intraperitoneal injection of AIP. This result show that AIP is a potent adjuvant.