Numerous efforts have been made to isolate immunologically active component from Mori Cortex Radicis, since it has been used in the treatment of bronchial asthma, and immune dis order in human. Recently, we reported the purification of an anti-allergic component of the Mori Cortex Radicis. Among the fractions we prepared in the previous study, a fraction was active in the proliferation of murine lymphocytes. The active component (HHM 3-1) was elucidated as a polysaccharade with a small amount of lignin. When it was subjected to MALDI-MS by using 3-hydroxypicolinic acid as a matrix, the molecular weight of the component was estimated as 792688.2dalton. Total hexose and protein content of the component were estimated as 62.6% and 0.51%, respectively and it was composed mainly of glucose, galactose and mannose. The remaining part of the component was estimated as ligin because of the characteristic functional groups in IR and UV spectra. Concomitant treatment of HHM 3-1 with known mitogens synergistically increased the proliferation of B-cells and T-cells.