Hot water extract of pine cone (PCE) of Pinus parviflora Sieb. et Zucc. dose-dependently suppressed both solid and ascites tumor cells transplanted into various mice. Acidic polysaccharides of PCE significantly increased the survival time of mice bearing ascites tumor cells, and activity increased with acidity. One of the four polysaccharide fractions obtained by NaOH extraction showed the most potent antitumor activity. This fraction significantly suppressed the growth of solid tumor cells, with occasional tumor regression and necrosis, and with little or no cytocidal effect on cultured tumor cells. All acidic polysaccharides were able to activate mouse macrophage-like cell line J774.1. There did not appear to be any correlation between the antitumor activity of these polysaccharides and their content of arabinose (or fucose), mannose, galactose, glucose, or uronic acid.