The blue mussel, Mytilus edulis, has been utilized extensively for monitoring in marine environments because of its tendency to bioaccumulate metals. Metal concentrations within soft tissues of the organism are often measured to estimate metal levels within a particular coastal area. The use of the organism's blood plasma as a biomonitor for heavy metal exposure has not been extensively studied. Metal ions are thought to be transported throughout the circulatory system by metal-transport proteins in the blood, where they are subsequently released to sites of metal accumulation within the organism. Although experiments in our laboratory using gel-filtration chromatography coupled with atomic spectroscopy support the existence of metal-protein complexes, low-molecular weight (LMW) compounds (<2 kDa) have also been identified as important chelators of metal ions within blood plasma. Metals associated with these LMW compounds include Ca, Cr, Fe, K, Mg, Mn, Ni and Zn. The compounds have been detected in mussels from four study sites along the New England coast. It is likely these LMW compounds have not been reported in the past because they are not detected using common protein detection techniques. Filtration and ammonium sulfate precipitation are simple methodologies useful for partially purifying these LMW compounds from the larger proteins in blood plasma previously reported to bind metals. The LMW compounds identified appear to be peptidic in nature based on the presence of numerous amino acids in their chromatographic fractions.