Photocatalytic sterilization of Escherichia coli (bacterium) or Saccharomyces serevisiae (yeast) was conducted with a rectangular bubble-column photoreactor (40 mm in width, 40 mm in breadth, and 250 mm in height) containing slurried TiO2 semiconductor particles. The profiles of cell deactivation with sterilization time could be expressed in fair agreement with experimental data, based on a series-event model and a second-order kinetics with respect to the concentrations of microbial cells and oxidative radicals generated by photoexcitation of TiO2 particles. Sterilization rate constants for the microbes were determined under various conditions of TiO2 concentrations (0-5 x 10(-1) kg/m(3)) and average light intensities (0-223 W/m(2)) in the photoreactor. Linear relationships were obtained between the rate constants and average light intensity at TiO2 concentration of 1 x 10(-2) kg/m(3). When incident light intensity was kept constant (27 W/m(2) for E. coli or 238 W/m(2) for S. cerevisiae), the correlations between the rate constants and TiO2 concentration were interpreted considering a fraction of TiO2 particles adhered to the cells in slurry.