The contamination of clean surgical wounds with anaerobic and aerobic bacteria was studied in 52 hip operations. In addition to wound samples, samples from air and the patients' skin were taken. The median of the total number of bacteria isolated from the wounds was 26 colony-forming units (cfu). The median percentage of anaerobic bacteria in the wound counts was 30. Propionibacterium spp. were found in 71 per cent of the wounds and anaerobic or microaerophilic cocci, most often Peptococcus spp., were found in 23 per cent. In six of 43 patients the same bacterial flora was seen in the skin samples and wounds. The geometric mean of the total number of bacteria in the air was 70.3 cfu/m3. Of these the median percentage of anaerobic bacteria was 30.3. When operating clothing of a disposable fabric (Barrier 450, Johnson & Johnson) was used, the counts of airborne bacteria were a little less than half those found when conventional cotton clothing was worn. Probably because of the overall low air counts in the operating theatre and the great variability in the individual bacterial counts from the operation wounds, a significant decrease in wound contamination was not observed. A positive correlation was found between the duration of operation and wound contamination.