The aquatic environment is known to contain a variety of natural and anthropogenic compounds that are capable of interacting with the genetic material of aquatic organisms. The increases in the levels of these anthropogenic contaminants, associated with widespread industrialisation, has led to the requirement for reliable methodologies to monitor their potential impact upon exposed aquatic organisms. Of the molecular techniques currently available, the 32P-postlabelling assay for the detection of DNA adducts offers considerable potential for the qualitative and quantitative assessment of genotoxin exposure. Here we describe several studies in which the technique was adapted for evaluation in two marine bioindicator species the common mussel Mytilus edulis and the flatfish Limanda limanda. Laboratory studies in which M. edulis specimens were exposed to 2-aminofluorene and 4-nitroquinoline 1-oxide confirmed the species' capacity to form genotoxin-related adducts. However, in further studies, no exposure-related adducts could be detected in M. edulis specimens placed in mesocosms containing environmentally realistic levels of anthropogenic contaminants. Biologically significant levels of adducts were detected in L. limanda specimens exposed to sediment bound contaminants under controlled conditions, although the levels did not appear to be statistically significant. An in situ study in which adduct levels were determined in L. limanda specimens from two sites of contrasting contamination levels proved to be more conclusive. The results were both biologically and statistically significant, suggesting that adduct levels could well be related to the levels of sediment-bound contaminants. Together the studies confirmed that the determination of the levels of DNA adducts could be used as indicators of the exposure of aquatic organisms to environmental genotoxins.