Controlled Smith degradation and limited hydrolysis of ginsenan PA, the main phagocytosis-activating polysaccharide isolated from the root of Panax ginseng C. A. MEYER, were performed. The reticuloendothelial system-potentiating and anti-complementary activities of the degradation products were investigated. Methylation analysis of the primary and secondary Smith degradation products indicated that the core structural features of ginsenan PA include a backbone chain mainly composed of β-1, 3-linked D-galactose. Almost half of the galactose units in the backbone carry side-chains composed of β-1, 6-linked D-galactosyl residues at position 6. Further 3, 6-branching of D-galactose units was observed in a part of the side-chains. α-L-Arabinose units are connected mainly to the core galactose moieties via position 6. Removal of most of the arabinose units had a considerable effect on immunological activity.