The effects of different exposure concentrations of palladium (Pd) on relative metallothionein (MT) response and bioaccumulation were investigated in zebra mussels (Dreissena polymorpha). The mussels were exposed to 0.05, 5, 50, and 500 microg/L Pd2+ for 10 weeks under controlled temperature and fasting conditions. Relative MT contents were assessed by a modified Ag-saturation method, which allows to discriminate between MT bound to Pd (Pd-MT) and MT bound to unidentified metals (Ag-MT). Determination of metal contents resulted from atomic absorption spectrometry following a microwave digestion. For unexposed mussels and mussels exposed to 0.05 microg/L Pd no metal accumulation could be detected. All other exposure concentrations resulted in detectable Pd accumulation in mussels with final tissue concentrations of 96 microg/g (500 microg/L), 45 microg/g (50 microg/L), and 9 microg/g (5 microg/L). Compared with initial levels Pd-MT concentrations at the end of the exposure period were 600 (500 microg/L), 160 (50 microg/L), and 27 (5 microg/L) times higher. These results show that an increase in MTs in D. polymorpha already occurs at relatively low aqueous Pd concentrations indicating that there is the need for detoxification of Pd in the mussel. Furthermore, correlations between Ag-MT and Pd accumulation indicate that higher exposure concentrations are associated with adverse effects on the mussels. Thus, harmful effects of chronic Pd exposure of organisms even in lowest concentrations cannot be excluded in the environment.