The pathogenicity in relation to encapsulation and potential for synergy between Neisseria gonorrhoeae and Bacteroides spp. was studied by subcutaneous and intraperitoneal inoculation of single organisms or mixtures of these organisms into mice. Ten isolates of N. gonorrhoeae and 12 isolates of Bacteroides spp. were studied. When injected alone, only heavily encapsulated organisms induced subcutaneous and intraperitoneal abscesses. However, N. gonorrhoeae did not induce intraperitoneal abscesses and did not survive in intraperitoneal abscesses even when inoculated with Bacteroides spp. Abscesses developed after subcutaneous inoculation of mixtures of heavily encapsulated Bacteroides and heavily or slightly encapsulated N. gonorrhoeae or of slightly encapsulated Bacteroides and heavily encapsulated N. gonorrhoeae. Heavily encapsulated N. gonorrhoeae did not survive in subcutaneous abscesses longer than 3 days after being inoculated alone and survived no longer than 7 days after inoculation with heavily encapsulated Bacteroides spp. An increase in the colony-forming units of Bacteroides spp. per abscess was observed when they were inoculated with N. gonorrhoeae. When mixed with heavily encapsulated isolates, slightly encapsulated N. gonorrhoeae or Bacteroides isolates became heavily encapsulated. These data demonstrate the effect of encapsulation on the relationship between N. gonorrhoeae and Bacteroides spp. and the potential for synergy between these organisms.