Before the early 1970s, a large proportion of samples collected from sites of postoperative wound infections yielded no pathogens upon routine culture yet did contain pleomorphic forms visible upon gram staining. The inability to recover pathogens often led to incorrect choices of antibiotics for empirical therapy and thus to clinical failures of therapy. It is now known that many of these infections were due to the various endogenous anaerobic constituents of the normal human microflora. Because of advances in the techniques used for anaerobic specimen collection and culture, anaerobic bacteria are now routinely recovered from a variety of intraabdominal and postoperative soft-tissue infections. In all but clean operative procedures, the causative organisms often reflect the normal aerobic and anaerobic flora of the resected organ. Before colonic surgery, counts of both aerobes and anaerobes must be reduced by appropriate mechanical cleansing and antibiotic administration. Successful treatment of surgical infections includes both the implementation of careful operative technique and the choice of appropriate antibiotics.