A plasmid determining resistance to erythromycin (inducible), lincomycin, and vernamycin B α was isolated from a strain of Streptococcus sanguis isolated from dental plaque of a patient who had been on erythromycin therapy for an extended period of time. The plasmid, designated pAM77, had a molecular weight of 4.5 × 10 6 and was present to the extent of one to two copies per bacterial chromosome. The plasmid could transform S. sanguis strain Challis.