Zinc pyrithione (ZnPT) and copper pyrithione (CuPT) are widely used antifouling booster biocides in the shipping industry as a replacement for the banned tributyltin (TBT). Sub-lethal effects of zinc pyrithione (ZnPT; 10 mu g L-1), copper pyrithione (CuPT; 10 mu g L-1) and combined copper-zinc pyrithione (CZPT; 5 + 5 and 10 + 10 mu g. L-1) to freshwater indicator mussel Unio crassus (5.49 +/- 0.85 cm in size) were determined by investigation of the total hemocyte counts, biochemical analysis mussel tissues and histopathological observations after 48-h and 7-d exposures. Mean total hemocyte counts differed significantly among all groups, for both exposure durations. In the 7 d, lipid peroxidation levels (malondialdehyde, MDA nmol g(-1) wet tissue) and reduced glutathione levels (GSH mu mol g(-1) wet tissue) were lower than 48 h. However, there were no significant differences among groups. Total protein levels (mg mL(-1)) were approximately in the same range and differed only among 7-d exposure groups. The gill tissues showed hemocytic infiltration and lipofuscin like structures, while digestive gland showed loss of digestive tubules in the treatments of pyrithione compounds. Histopathological damages were most severe in the combined exposure groups. In conclusion, ZnPT, CuPT and combined CZPT(5/10) are highly toxic on a freshwater indicator mussel species U. crassus.