The aim of this study was to (a) evaluate the potential for the 'Comet assay' to be used as a method for detecting genetic damage in the common cockle Cerastoderma edule; and (b) to compare the relative sensitivity with Mytilus edulis as the bivalve widely used as a sentinel species in biomonitoring studies. In vitro validation studies were carried out on haemocytes from each species using hydrogen peroxide (H2O2), a known oxidant and the induced DNA damage was measured using the Comet assay. On exposure to 0, 100, 500 and 1000 microM H2O2, a significant concentration-dependent increase was observed in both species. Use of an additional concentration of 5000 microM H2O2 showed that while DNA damage could be assessed in M. edulis at this concentration, only a few cells from C. edule were amenable to measurements owing to extensive DNA damage ("hedgehog cells"). The evidence also suggested that the cells from C. edule are more sensitive to oxidative damage induced by H2O2 when compared with M. edulis. Bearing in mind that sediments are the ultimate sink for many contaminants, this study demonstrates the potential application of sediment-dwelling C. edule as a useful biomonitoring species.