Wound infection was prospectively studied in 7,519 consecutive operations after preoperative classification as clean, clean-contaminated, and infected. The overall infection rate was 3.9 per cent. Clean, 3.2 per cent; clean-contaminated, 4.4 per cent; contaminated, 12.4 per cent; infected, 16.2 per cent. Wound infection was not seasonally related or dependent on changes in house staff. In clean cases, the predominant role of Staphylococcus aureus (37%) has been superceded by enterococci (44%). In clean-contaminated cases, enterococci (43.5%) were the most common, followed by Escherichia coli (40.0%). In contaminated wounds, E. coli was most common (40.0%). The infected case category grew mixed flora (E. coli, 82 per cent; enterococci, 54 per cent, and Pseudomonas aeruginosa, 43 per cent). Nosocomial organisms were important only in the contaminated (14%) and infected (43%) categories. Antibiotic therapy before cultures are available should include agents with activity against enterococci as well as S aureus, and E. coli in clean cases.