Thiobacillus-ferrooxidans immobilised in biomass support particles with activated carbon coating were used in a packed-bed bioreactor to study the combined effects of chemical and biological catalysis on the oxidation of ferrous iron. The effect of ferrous iron concentration (in the range 530 kg m(-3)) and of its volumetric loading on the kinetics of reaction were investigated. With low concentrations of ferrous iron, 5-10 kg m(-3), the combined catalysis did not offer a significant advantage to oxidation of ferrous iron and the kinetics of reaction were slightly faster than those achieved with just the biological catalyst. With ferrous iron at a concentration of 20 kg m(-3), the combination of chemical and biological catalysis resulted in a remarkable enhancement of the reaction rate. The maximum oxidation rate of ferrous iron in the presence of combined catalysts, 21.9 kg m(-3) h(-1), was twice as high as that achieved with just the biological catalyst.