The in vivo immunomodulatory function of the activity of murine natural killer (NK) cells induced by high mannuronic acid-containing alginate (HMA) was examined. HMA was injected i.p at doses of 25 and 100 mg/kg. The NK activity was 3 times higher with 100 mg/kg HMA than the baseline. In addition, in vitro studies of splenocytes cultured with HMA for 20 h showed a significant increase in NK activity at E:T ratio of 100: 1; a 160$\%$ and 210$\%$ increase at 10 and 100 $mu$g/mL, respectively. There was a six fold increase in interferon-$gamma$ production in a postculture of splenocytes with 100 $mu$g/mL HMA. HMA had no suppressive effects on the lymphocyte function in the presence or absence of mitogens. This suggests that HMA is useful in cancer immunotherapy.