Digestive cell lysosomes in mussels are known to respond to individual organic chemicals and metals after experimental exposure under laboratory conditions but reports dealing with the response to mixtures of pollutants are scarce. The aim of the present investigation was to compare the lysosomal responses elicited by exposure to a model organic chemical compound (benzo(a)pyrene, B[a]P), a model toxic metal (Cd) and their combination (B[a]P+Cd) under controlled laboratory conditions. Dimethylsulfoxide (DMSO) was used as vehicle to dissolve organic chemicals into seawater. Control mussels were either kept untreated in clean seawater or treated with DMSO. Digestive glands were excised on Day 21. beta-Glucuronidase activity was demonstrated in 8 mum cryotome sections. Lysosomal volume, surface and numerical densities (Vv, Sv and Nv), and surface-to-volume ratio (S/V) were quantified by image analysis. Lysosomal enlargement was evident in digestive cells of mussels exposed to either Cd, B[a]P or B[a]P+Cd. Such enlargement was more marked after exposure to B[a]P+Cd than to B[a]P, but did not reach the levels recorded after Cd exposure. It seems therefore that the presence of B[a]P reduced to some extent the effects of Cd on digestive cell lysosomes in mussels.