Cells of Enterobacter sp. 4-2 which could remove large amounts of nickel ion (Ni2+)from aqueous solution were immobilized on magnetites. The optimal conditions for the adsorption of bacterial cells onto magnetites were a cell:magnetite ration greater than 1:10 (dry weight basis), Tris buffer with pH less than or equal to 6.5, and temperature at 25 degrees C. Under selected conditions (cell:magnetite ratio of 1:10, pH 6.0, and temperature at 25 degrees C), more than 90% of the bacterial cells were immobilized onto magnetites. The optimal conditions for immobilized cells to remove Ni2+ from aqueous solution were Tris buffer at an alkaline pH, the temperature higher than 37 degrees C, and the retention lime longer than 30 min. Under selected conditions (temperature at 25 degrees C and retention rime of 10 min), 50-90% of Ni2+ was removed by immobilized cells from the buffer at pHs 7-9. Complete recovery of adsorbed Ni2+ from immobilized cells was achieved by washing the Ni2+-cell-magnetite complexes with diluted citric acid.