Surgical infections are almost always polymicrobial, yet the critical importance of bacterial mixtures in these infections has received relatively little attention. The convincing data on the prevalence of mixed infections in surgery are reviewed. Both clinical and experimental evidence indicate that true synergy between certain aerobes and anaerobes may exist. Of the possible mechanisms of synergy, the most important seems to be the ability of anaerobes, their metabolic products, or their capsules to inhibit phagocytosis of aerobes by leukocytes. Other mechanisms of importance in special microbial combinations include provision of essential nutrients such as vitamin K, succinate, and various growth factors by one microbe to the other; alteration of local environment, including reduction of the oxygen tension and lowering of redox potential; and the provision of substances toxic to the host that permit species of bacteria to flourish concurrently. Further study of these interactions will shed light on the causes and correction of treatment failure.