Bacteria air samples were taken in the operating rooms, with no people present, to specify the level of air contamination and suggest bacteriological standards for different operating rooms. In the first step of this study, for 5 months the air contamination mean value of operating rooms, ventilated at 15 changes/hour was 18.5 Cfu/m3 +/- 1.9. In the second part of the study, during two years, 1 381 air samples were taken in 8 different operating rooms. The mean values of air contamination range from 1.4 Cfu/m3 in a Charnley isolator system to 121 Cfu/m3 in an operating room ventilated at 7.5 changes per hour. As a general rule, the airborne contamination is more significant in the oldest operating rooms than in the new one with filtrated air. The variations observed between 1981 and 1982 are explained by technical modifications of the system or progress in control of operating room conditions. Measurements of the bacterial contamination of the air give useful informations, but it is however better to ensure that the specifications for volume air supply and positive air pressure in the operating theatres are being fulfilled. The airborne bacterial concentration in a modern ventilated operating room should not exceed 30 Cfu/m3.