Summary A mixed infection in mice involving a penicillin-susceptible pathogen, Fusobacterium necorphorum and strains of either Bacteroides melaninogenicus or Bacteroides aralis was resistant to penicillin therapy when penicillin-resistant Bacteroides strains were used but not when penicillin-susceptible strains were used. The ability of penicillin-resistant strains of Bact. melaninogenicus and Bact. oralis to protect F. necrophorum from penicillin in vivo appeared to be due to β-lactamase production by these strains. In vitro β-lactamase activity could be demonstrated in Bacteroides strains which protected F. necropharum in vivo, but not in strains which failed to give protection.