Of 15 Peptostreptococcus sp. strains isolated between 1975 and 1984, 13 hybridized with the Tet K, Tet M, or Tet O determinant. A donor Peptostreptococcus anaerobius strain carrying the three determinants could transfer Tet M to P. anaerobius and Fusobacterium nucleatum recipients but not to an Enterococcus faecalis recipient, while neither Tet K nor Tet O was transferred.