Two plasmids designated pOB1 and pOB2 were isolated from Streptococcus faecalis strain 5952 and found to have molecular weights of approximately 46 X 10(6) and 28 X 10(6), respectively. pOB1 was found to determine hemolytic activity and was transmissible, whereas pOB2 appeared to determine a bacteriocin that is specifically inhibitory to S. faecalis strains harboring the 26-megadalton plasmid pAM539.