Little is known about the immunotoxic effects of sublethal mercury (Hg) exposure in bivalves, and whether or not stimulation of the immune system exacerbates Hg toxicity. The mussel, Mytilus edulis, was exposed to a total inorganic Hg concentration of 50 microgl(-1) as HgCl2 for up to 11 days compared to no added Hg controls. Then the immune functions of haemocytes, haematology, biochemistry and organ histology were assessed. Selected experiments were then repeated in mussels injected with lipopolysaccharide (LPS) to stimulate the immune system prior to Hg exposure. Hg exposure alone caused Hg accumulation mainly in gills and digestive gland, and to a lesser extent in the adductor muscle and haemolymph. This general pattern of Hg accumulation also occurred during the Hg plus LPS experiments. Hg exposure alone caused a transient increase of phagocytosis, but no Hg-dependent effects on neutral red retention or the cytotoxicity of haemocytes. Changes in haemolymph Na+, K+ and glucose levels were negligible during Hg exposure alone compared to controls. However, histological examination showed tissues injuries consistent with inflammation in the gills, digestive gland and adductor muscle during Hg exposure alone. LPS injection had a significant effect (Kruskal Wallis, P=0.02) on circulating haemocytes numbers at the end of the experiment compared to saline injected controls and the Hg+LPS treatment (mean+/-S.E.M., n=6, cells ml(-1)x10(6)); control, 2.1+/-0.08; LPS treated, 3.18+/-0.36, and LPS+Hg, 2.1+/-0.2. The Hg+LPS treatment also caused a transient decline in superoxide dismutase activity in haemocytes (85% decline, Kruskal Wallis, P=0.006) compared to controls. Target tissue inflammation and injury was greatly increased by the effect of Hg exposure with an LPS injection compared to either treatment alone. We conclude that Hg can cause inflammation in mussels in vivo, and that stimulation with LPS can greatly increase Hg-dependent immunotoxicity.