Mussel gill cells are attractive models in ecotoxicological studies because gills are the first uptake site for many toxicants in the aquatic environment; gill cells are thus often affected by exposure to pollutants. Our aim was to characterize mussel gill cells in vivo and in vitro by using morphological, histochemical and functional end-points. In paraffin sections stained with haematoxylin-eosin, three zones were distinguished in the long central gill filaments: frontal, intermediate and abfrontal. Various types of ciliated cells were present in the frontal zone, and both ciliated and non-ciliated cells were found in the abfrontal zone. The intermediate zone was comprised of flattened endothelial cells. Lipofuscin granules occurred in the three zones in variable amounts, depending on the specimen. Haemocytes were found in the haemolymph sinus of gill filaments. Mucocytes were identified in both frontal and abfrontal zones by means of periodic acid Schiff-alcian blue (PAS-AB) staining. In cryostat sections, succinate dehydrogenase (SDH) activity was mainly found in ciliated cells, whereas neutral lipids and acid-phosphatase-reactive lysosomes were present in all portions of the gill filament, mostly being related to lipofuscin granules. In mussels exposed to 5'-bromo-2'-deoxyuridine in vivo, proliferating cells were scattered throughout the gill filament. Gill cells (typically 2x10(7) cells/ml per mussel; 95% viability) were isolated by dissociation with dispase. Gill cell suspensions were heterogeneous: 58% were ciliated epithelial cells (positive for SDH), 42% were non-ciliated cells (including epithelial cells and haemocytes), 2.3% were mucocytes (positive for PAS-AB) and 4.25% were haemocytes (able to phagocytose neutral red-stained zymosan). Gill cell cultures were maintained up to 18 days without changing the culture medium, viability decreasing below 50% at day 18. Primary cultures of mussel gill cells might therefore be useful models for the in vitro assessment of xenobiotic impacts on coastal and estuarine ecosystems.