Ultraviolet irradiation has been employed in operating rooms for more than half a century in attempts to reduce airborne bacterial contamination. Safety considerations have limited its intensity to 25-30 mu w cm-2 and at this level no more than a fourfold reduction has resulted. In recent studies intensities up to 300 mu w cm-2 have been used without untoward effects, and, at the highest intensity, contamination as low as that obtained with ultraclean air ventilation systems was obtained. However this was only achieved in an operating room where the level of airborne contamination before the introduction of the radiation was already much lower (around one-fifth) than that usually observed, and the reduction attributable to the radiation was still only about 12-fold.