Microbiological cultures of specimens of tissue and of fluids from the wound in forty patients who had had consecutive clean, elective orthopaedic operations (excluding total joint replacements) and had not received antibiotics preoperatively were analyzed. Of the forty patients, twenty-three (58 per cent) had a positive culture on at least one of the media that were used and seventeen (43 per cent) had negative cultures. Of the forty specimens that were obtained from swabbing of the wound, eight (20 per cent) were positive on culture, compared with twenty (50 per cent) that were obtained from biopsy of tissue. Of these twenty-eight positive cultures, thirteen (46 per cent) were on routine blood-agar plates and fifteen (54 per cent), in broth only. Of the thirty-three bacterial organisms that were identified in the twenty-eight positive cultures of the wound, nineteen (58 per cent) were coagulase-negative Staphylococcus; eight (24 per cent), Propionibacterium acnes; two (6 per cent), Peptostreptococcus; and four (12 per cent), miscellaneous organisms. In all of the positive cultures on the blood-agar plates, except in those showing Propionibacterium acnes, there were five colonies or fewer. One patient had a clinical infection with Staphylococcus aureus that developed later, but the initial cultures of the wound had been positive for Staphylococcus epidermidis only. None of the bacteria that grew on culture were Staphylococcus aureus or the less common pathogenic gram-negative bacteria, such as Escherichia coli, Pseudomonas, or Klebsiella.(ABSTRACT TRUNCATED AT 250 WORDS)