Alginates are polysaccharides with gel-forming properties composed of 1,4-linked $eta$-D-mannuronic acid (M), $alpha$-L-guluronic acid (G), and alternating (MG) blocks. The M-and the MG-blocks, but not the G-blocks, have been known to be the active components of the alginates in experimental models. In this study, we have examined the ability of high M-alginate to activate immune cells. Alginate induced the macrophage anti-viral activity and the lymphocyte blastogenesis, and enhanced cytotoxicity of natural killer cell. In addition, alginates stimulated the macrophages to induce the production of $H_{2}O_{2}$, whereas alginates had no effect on NO production and suppressed the production of TNF-$alpha$. These findings suggest that high M-alginate may be modulating various elements of the host immune response.