The kinetics of continuous oxidation of ferrous iron by immobilized cells of Thiobacillus ferrooxidans was studied in a packed-bed bioreactor. Polyurethane foam biomass support particles were used as carriers for cell immobilization. Effects of ferrous iron concentration and its volumetric loading on the kinetics of the reaction were investigated. Media containing different concentrations of ferrous iron in the range 5-20 kg m(-3) were tested. For each medium the kinetics of the reaction at different. volumetric loadings of ferrous iron, at a constant temperature of 30 degrees C, were determined. With media containing 5 kg m(-3) and 10 kg m(-3) Fe2+, the fastest oxidation rates of 34.25 kg m(-3) h(-1) and 32 kg m(-3) h(-1) were achieved at a dilution rate,of around 6 h(-1), which represents a residence time of 10 min. Employing a higher concentration of ferrous iron (20 kg m(-3)) in the medium resulted in lower oxidation rates, with a maximum value of 10 kg m(-3) h(-1), indicating an inhibitory effect of ferrous iron on growth and activity of T. ferrooxidans. The reliable performance of the bioreactor during the course of the experiments confirmed the suitability of polyurethane foam biomass support particles as carriers for T. ferrooxidans immobilization.