Microbial reduction and recovery of palladium was achieved at 25 degrees C and pH 7 using resting cells of the metal ion-reducing bacterium Shewanella algae when either lactate or formate was provided as an electron donor. The bioreductive recovery of palladium with formate was a fast process: 1-10 mol/m(3) aqueous Pd(II) ions were completely reduced to elemental palladium within 60 min. The biogenic precipitates in the S. algae cells were crystalline Pd(0) nanoparticles of less than 10 nm in size. The specific reduction rate of Pd(II) ions reached a maximum of 2.14 x 10(-14) g-Pd/cell/min when the initial concentration of aqueous PdCl(2) solution was 5 mol/m(3). The recovery capacity of S. algae cells was determined as 1.49 g-Pd/g-dry cells, indicating that the 10 mol/m(3) aqueous PdCl(2) solution was concentrated up to 570-fold by the microbial deposition in the bacterial cells. The biogenic Pd(0) nanoparticles were released from bacterial cells into aqueous solution within 30 min, by ultrasonic treatment of cell suspension with addition of NaOH.