The rhamnan sulfate extracted from green algae seaweed, Monostroma nitidum was characterized as activity in vitro culture assay with macrophages from mice. Rhamnan sulfate indicated that F-4-3 fraction enhanced glucose consumption, as well as the production of nitrogen dioxide and tumor necrosis factor(TNF). F-4-3 fraction was also augmented IL-1 secretion from those macrophages. Effects of the pretreatment of peritoneal macrophages with rhamnan sulfate F-4-3 fraction and several polysaccharides as relative standard on the production of H₂O₂ induced with unopsonized zymosan A were examined. Pretreatment with polysaccharides inhibited the zymosan A mediated H₂O₂ production by macrophages. The phorbol myristate acetate (PMA) mediated H₂O₂production was not affected by the pretreament. These result suggested that pretreatment of rhamnan sulfate interfered with the interaction of macrophages zymosan A. Rhamnan sulfate inhibited zymosan A mediated production of H₂O₂ by macrophages and F-4-3 fraction was also activator of macrophages.