Although enterococci are relatively resistant to penicillin, the mechanism of resistance is largely unknown and enzymatic inactivation does not play a role. In this study, an isolate of Streptococcus faecalis was found to have beta lactamase activity resulting in complete inactivation of penicillin. With a high inoculum, this strain was resistant to greater than 1,000 micrograms/ml of penicillin. Penicillin resistance and beta lactamase activity were transferred by conjugation at a high frequency to an enterococcal laboratory recipient strain together with two plasmids of molecular size 34 X 10(6) and 56 X 10(6), thus demonstrating the emergence of plasmid-mediated penicillin resistance in the genus Streptococcus.