Certain foreign materials have been demonstrated to enhance the infectivity of aerobic and anaerobic bacteria. Whole blood and other protein compounds encountered in surgical settings or trauma were tested for their effect on infectivity of nonsporeforming anaerobic bacteria. Infectious synergistic mixtures of Bacteroides fragilis plus Peptostreptococcus anaerobius and Bacteroides melaninogenicus plus Fusobacterium necrophorum were each diluted to a barely noninfectious or minimally infectious concentration (subinfective inoculum) that was injected intraperitoneally into mice alone and in combination with test proteins. Infectivity was measured by deaths from sepsis or abscess(es) within the abdominal cavity at autopsy at 1 week. Two hemostatic agents, Gelfoam powder and Avitene (final concentrations, 10 mg/ml), and crystalline hemoglobin (4 g/100 ml) each produced a marked increase (P < 0.001) in the rate of infection when mixed with a normally subinfective inoculum of either bacterial mixture. Fresh homologous mouse blood (0.25 ml) injected intraperitoneally without anticoagulant also significantly enhanced infectivity (P < 0.01) of a subinfective inoculum of B. fragilis plus P. anaerobius. These studies demonstrated the capacity of whole blood, hemoglobin, and hemostatic agents to enhance the infectivity of certain nonsporeforming anaerobic bacteria. The high concentrations of anaerobic bacteria in the gastrointestinal, female genital, and respiratory tracts produce an increased risk of human infection after surgery or trauma in these sites; the protein agents studied here may further enhance infection.