It has been reported that mycelia of the Cordyceps sinensis (CS) can function as an immunostimulant. However, the active constituents of the mycelia are not well known. In this study, we investigated which components of the mycelia of CS induce monocyte activation and then structurally analyzed the active components. Assay of the effect of crude-(CS-P), soluble-(CS-Ps) and insoluble-(CS-Pp), polysaccharides extracted from the mycelia of CS, on macrophage production of TNF-alpha, indicated that CS-Pp enhanced TNF-alpha production to the highest extent. Furthermore, Structural analyses demonstrated that CS-Pp is a 1,3-beta-D-glucan contained some 1,6-branched chains and the mean particle diameter is 1.5 mum.