Two operating rooms (OP K I, OP K II) with conventional air-conditioning and one operating room with horizontal laminar-flow-ventilation (TAVS) were compared by measurements of airborne microorganisms, settling microorganisms and wound contamination. In OP K I and OP K II the number of airborne colonie forming units (cfu) was about 8/m3 when the rooms were empty and between 70/m3 (OP K I) and 140/m3 (OP K II) during operations. The first air in the TAVS-OP contained less than 1 cfu/m3, downstream of the operating team up to 80 cfu/m3. The number of settling microorganisms at the wound site was about 13 cfu/100 cm2 h in OP K I and OP K II, and about 2 cfu/100 cm2 h in the TAVS OP. Wound swabs showed a contamination rate of 45% (OP K I) and 54% (OP K II) versus 31% under TAVS-conditions. The difference is statistically significant with p less than 0.05. Various other factors as e.g. the duration of exposition, traffic of persons into and out of the rooms and the fate of microorganisms after sedimentation into the wound are discussed. While the number of settling germs is growing in linear proportion to the duration of exposition, the number of wounds found contaminated does probably approach a steady state in dependence of sedimentation rate and die-off rate in the wound.