A Biap slit sampler and a Casella Mk 2 slit sampler were studied in an orthopaedic operating theatre. Both showed similar bacterial counts in the range of 74-640 c.f.u. (colony forming units)/m3. During operations, as average count of 220 c.f.u./m3 (range 52-482) was obtained. A close relationship was demonstrated between the number of airborne bacteria and the frequency of traffic through doors.