Palladium uptake by resting cell suspensions of Desulfovibrio desulfuricans NCIMB 8307 was studied without or with electron donor (formate), which gave metal uptake attributable to biosorption of Pd(II) and bioreduction of Pd(II) to Pd(0), respectively. The maximum biosorption capacity of palladium (at pH 2) was up to 196mg Pdg(-1) dry cells (1.85mmolg(-1); approx 20% of the dry weight). Biosorption was to 85% of the maximum in less than 10min and the biomass was saturated within 30min. Biosorption of Pd(II) was greater from the chloro- than the ammine complex and was)2 inhibited in the presence of excess chloride ion. Bioreductive accumulation of Pd(11) from Pd(NH3)(4)(2+) was achieved in the presence of electron donor (formate) but was also inhibited by excess Cl-. Up to 100% of Pd(II) reduction to Pd(0) was achieved within 5 min anaerobically at pH 7 and 30 min at pH 3. Pd(0) was localized on the biomass surface using electron microscopy and was characterized using energy dispersive X-ray microanalysis (EDAX) and X-ray diffraction analysis (XRD). Biosorption was Pd-specific with respect to Pt and Rh using test solutions and acid (aqua regia) leachates from spent automotive catalysts. The total Pd removed from the latter was only 15%, attributable to the inhibitory effect of residual chloride ion from the acidic extractant. Pd biorecovery is limited by the need for an improved extraction technology to minimize the formation of PdCl42- in solution rather than by constraints of the Pd-accumulating biomass.