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The potential for synergy between aerobic, facultative, and anaerobic bacteria was studied by subcutaneous inoculation of mixtures of these organisms into mice and observation of subsequent mortality and abscess formation. The anaerobic bacteria tested included 12 strains of gram-positive cocci and two strains each of Bacteroides species, Clostridium species, and Fusobacterium species. The facultative and aerobic bacteria included one strain each of Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli, Klebsiella pneumoniae, and Proteus mirabilis. Mortality increased significantly when each aerobic organism was inoculated along with either of the Bacteroides species. A similar increase occurred when the anaerobic gram-positive cocci were inoculated along with P. aeruginosa (four of six combinations) or S. aureus (four of six). The rate of abscess induction increased significantly when 10 of the 12 strains of anaerobic gram-positive cocci were injected along with B. fragilis and when nine of these strains were inoculated along with Bacteroides asaccharolyticus . The results demonstrate synergistic potential between Bacteroides species and all aerobic bacteria tested, between Bacteroides species and most anaerobic gram-positive cocci, and between most anaerobic gram-positive cocci and P. aeruginosa or S. aureus.