Polysaccharides were extracted from mycelia of Phellinus linteus grown under different culture conditions. The in vitro immunostimulating activity was measured by plaque-forming cell (PFC) assay. The activity of the polysaccharides was different from that of mycelia from which was extracted. The number of PFC's ranged from 40 to 600 depending on the media. When P. linteus was cultured on a medium with mannose or starch as a sole carbon source, the fungus produced polysaccharide with the highest activity of 960 PFC. Activity was therefore increased by $50%$ compared with polysaccharide which was extracted from mycelia grown on medium with glucose. pH had little effect on the change in activity. All polysaccharides on media with different pH stimulated about 600 PFC. These results suggest that activity could be increased by polysaccharide modification through changes in physiological conditions.