The aim of this preliminary study was to investigate the presence of proteins in the tissue fluids of the sea mussel Isognomon alatus. Protein extraction was done by the chloroform-cold ethanol technique. Immunization for production of antibody to be used as reagents in Western blotting, assessment of the protein concentration by the Bradford method, protein characterization by native polyacrylamide gel electrophoresis (PAGE) were also performed as a part of the methodology of this study. The results showed a protein content of 65 mg/ml in tissue fluids and a protein band approximately of 220 kDa in PAGE that was further confirmed by the Western blotting. Future work should investigate the structure and function of the proteins separated from the tissue fluids and we considered it as a limitation of this investigation. The sea bivalve literature is scanty. However the limitation of this work we still can conclude that there are high molecular weight proteins in large concentrations in tissue fluids of the sea mussel Isognomomalatus.