Abstract: Awater soluble polysaccharide has been isolated from clitopilus caes-pitosus PK. and its major constitution is found to be glucose mixed with mannose and a small quantity of fucose.An unitary glucan with the average mol, wt. 1320000 has been obtained through fractionating with methanol and further DEAE-cellulose column chromatograph. By means of Smith degradation, periodate oxidation, G.L. C., G.L.C.-M.S., I. R., methylation analysis etc, the structure of the glucan has been identified as a highly branched one with (l→3) linkages in its main chain and with (l→6) and a few (l→4) linkages in the side chain. In average for every five glucosyl residues in the main chain there are two 6-position branched points, each of which is linked to one glucosyl residues at 1-position. Asmall quantity of (1→4) linkage stands on peripheral region. The basic structure can probably be expressed by the following formula: where Grepresents D-glucopyranose, and n =0, 1, 2, 3.It has been found that polysaccharide shows some extent (42.8%) of tumor inhibition.