In order to determine if opsonin-like molecules exist in the coelomic fluid of sea cucumber Apostichopus japonicus and identify their properties, immunochemical methods including phagocytosis assay, SDS-PAGE, gel filtration and mass spectrometric analysis were performed. The phagocytosis of sea cucumber phagocytes was quantified in vitro using heat-killed yeast cells as target cells. Phagocytes of the sea cucumber were capable of phagocytosis in isotonic buffer, cell-free coelomic fluid (CFCF) and yeast cell-adsorbed CFCF, but the phagocytosis was significantly enhanced when the incubation medium was CFCF. This phagocytosis promoting activity of CFCF was eliminated by pre-adsorption of CFCF with yeast cells. SDS-PAGE analysis showed that a protein with 18 kDa was able to bind to yeast cells. By using gel filtration, the protein was isolated from coelomic fluid of the sea cucumber. Functional test of this molecule demonstrated that it was a phagocytosis-enhancing opsonin-like molecule. However, the phagocytosis-enhancing function of the opsonin-like molecule was dependent on some unknown factor(s) in the CFCF, which needs further investigations. Mass spectrometric analysis indicated that this opsonin-like molecule is a new protein.