The microbicidal effect of ozone in air was tested at concentrations between 50 and 600 micrograms/m3 against the species: Staphylococcus epidermidis, Micrococcus luteus, Arthrobacter citreus, Bacillus subtilis (veg.), Escherichia coli, Salmonella typhimurium, Serratia marcescens, Pseudomonas fluorescens and Candida albicans. The microorganisms were exposed on membrane filters at 60-75% relative humidity and 21.5-22.5 degrees C. After exposure times between 1 min and 60 min, the filters were incubated on appropriate agar media. The effect of ozone was determined by comparing the number of colonies on exposed filters to the number on nonexposed filters. The die-off curves (colony count against time) proved not to be rectilinear, but to become steeper with increasing time of exposure. Furthermore, the velocity of reduction increased more than proportional with increasing concentration of ozone. Therefore, the bacterial decay seems not to follow first order reaction kinetics. The values presented for k (constant of the velocity of die-off) and D (decimal reduction time) are valid only for narrow ranges of the initial part of the exposure. Concentrations of 50 to 100 micrograms (0)3/m3 for 1 h resulted only in little reduction, whereas 500 to 600 micrograms/m3 for one hour led to 99% reduction in all bacterial species tested. The gram-positive species seemed to be more sensitive than the gram-negative species, C. albicans proved to be more resistant than the bacteria.