There is growing evidence that contaminants may be partly responsible for the observed increase in disease in marine organisms by adversely affecting their immunity. Bivalve molluscs are common sentinels used in invertebrate immunotoxicology, however, to date, studies have been restricted to a few resilient species. This present study is a comparative investigation into the effects of the polycyclic aromatic hydrocarbon, phenanthrene, on the immunocompetence of three bivalve species. The commonly-studied marine mussel, Mytilus edulis, was compared with two species that have never been studied with respect to immunomodulation, namely, the edible cockle, Cerastoderma edule and the razor shell, Ensis siliqua. Animals were exposed to a range of phenanthrene concentrations (50, 100, 200 or 400 microg l(-1)) and haemocyte immune parameters, including haemocyte counts, phagocytosis, superoxide generation, lysosomal enzymes and lectin-binding, were monitored. Aims were not only to extend existing knowledge of bivalve immunotoxicology, but also to establish whether contaminant-induced immunomodulation in the sentinel species, M. edulis, is comparable to that observed in other bivalves. Results showed that the immune response of the three species was differentially affected by phenanthrene exposure, with immunomodulation in M. edulis not reflecting the immunological changes observed in the other two species. This suggests M. edulis may not be a suitable sentinel bivalve, and that other species, such as C. edule, may more accurately reflect the general immunological response of this group of marine animals.