In eight hospitals the quantity of microorganisms in the supply air of air conditioning systems was determined. A Reuter-Centrifugal-Sampler (by Biotest, Frankfurt) sampled 160 litres of air in four minutes and blew the particles with microorganisms on a culture medium (GKA-medium, by Biotest, Frankfurt). The Agar was breeded for 48 hours at 37 degrees C. The colonic-forming-units were counted and transformed to a volume of a cubic meter (CFU/m3). - In other publications you can find determinations of microorganisms in the supply air for operating theatres up to 15 CFU/m3. We found up to 40 CFU/m3 in the supply air after three step filtration (with an high efficiency filter) and 20-85 CFU/m3 after two step filtration. - The recommendations of other authors and some directives say that in the supply air up to 10 CFU/m3 are acceptable. According to our experiences the supply air should not contain more than 10 CFU/m3 after a 3-step-filtration and not more than 50 CFU/m3 after a 2-step-filtration. Otherwise the function of the air conditioning system should be controlled and the maintenance, too. - We think that according to the different requirements in hygiene a supply air with less than 10 CFU/m3 in the mean allows an unlimited use of the air conditioned room, while mean values up to 50 CFU/m3 are acceptable for normal operating rooms and other rooms, except for operating rooms with special hygienic requirements as for example for transplantation, arthroplasty, heart-operations (corresponds to II. and III. classification of rooms, DIN 1946/4). - Values up to 150 CFU/m3 are acceptable for rooms with normal hygienic requirements for supply air (III. classification of rooms, DIN 1946/4) and for labour-, intensive-care, newborn- and infant rooms.