In order to study the immune mechanism of coelomic fluid of Strongylocentrotus intermedius,we researched the phagocytosis activity in different media and different treatments of yeast cell.Then we analysed the non-cell coelomic fluid and ingredient which was isolated from regulating yeast cells and non-regulating yeast cells with SDS-PAGE,and the non-cell coelomic fluid was dialysed,SephadexG-200 gel chromatography,and SDS-PAGE;In isotonic buffer and cell-free coelomic fluid by yeast cell adsorption,we determined the phagocytosis effected by opsonin-like molecule of different concentration and different time.The result showed that isotonic buffer,cell-free coelomic fluid by yeast cell adsorption and non-cell coelomic fluid all can regulate phagocytosis,and the ratio was 1.00:1.11:1.94,and the phagocytosis rate of regulating yeast cells was 1.61 times of non-regulating yeast cells in isotonic buffer;There was a new opsonin-like molecular band from ingredient which was isolated from regulating yeast cells,and the molecular weight is 250.62 ku.We also found the molecular weight is 250.62 ku after dialysis,SephadexG-200 gel chromatography;In tested concentration range,the greater the opsonin-like concentration,the higher the phagocytosis and 5.0 and 10.0 μg/mL groups were higher than control group in each test time.In isotonic buffer,10.0 and 5.0 μg/mL groups had very significant difference(P<0.01)compared to control group in 30 min after reaction.In cell-free coelomic fluid by yeast cell adsorption,10.0 μg/mL group had significant difference compared to control group(P<0.05)in 30 min after reaction.These results indicate that there is a kind of new opsonin-like molecule in coelomic fluid of S.intermedius and the molecular weight is 250.62 ku which can promote phagocytosis.