Mixed infections with three Fusobacterium species and seven other bacterial species were studied in a subcutaneous abscess model in mice. Fifteen Fusobacterium isolates (eight F. nucleatum, four F. necrophorum, and three F. varium) and one isolate each of Bacteroides fragilis, B. asaccharolyticus, Staphylococcus aureus, Group A beta-haemolytic streptococcus, Escherichia coli, Klebsiella pneumoniae and Pseudomonas aeruginosa were studied. Electronmicrographs showed the presence of a thin mucopolysaccharide wall before and after inoculation into mice in 12 isolates which included all of 11 Fusobacterium isolates that induced subcutaneous abscesses. After co-inoculation of Fusobacterium isolates with other species and selective therapy with antimicrobial agents, S. aureus and K. pneumoniae were found to be of equal or greater importance in abscess induction than were Fusobacterium isolates, while Fusobacterium isolates were found to be more important than Group A streptococci and E. coli. Mutual enhancement of the numbers of organisms in mixed infections was observed with Fusobacterium spp. and K. pneumoniae, P. aeruginosa or Bacteroides spp. Suppression of Fusobacterium spp. was noticed only when they were co-inoculated with Group A streptococci. The additive or synergistic capabilities of Fusobacterium species highlighted their potential pathogenicity in infection.