Clinical trials carried out in Britain, Europe, and the United States have confirmed that between 80 and 90% of bacterial contaminants found in the wound after surgery come from colony forming units (cfu) present in the air of the operating theatre. When the numbers of cfu are reduced, there is a corresponding reduction in the incidence of wound sepsis. The sources, concentration, and movement of airborne cfu during an operation are identified, and various ways of reducing their numbers are reviewed. Only reduction of cfu in the operating zone to less than l/m 3, by means of an exponentially curved flow of microbiologically clean air, together with a total body exhaust system worn by the members of the surgical team and the patient, provides aseptic conditions without imposing restrictions or changes in surgical procedure.