Mutants resistant to penicillin G were selected in a stepwise manner from nine different species of enterococci. Mutants with the highest level of resistance showed cross-resistance to all beta-lactams tested. For eight of the nine species, resistance correlated with increased production of a low molecular weight penicillin-binding protein (PBP). Two of these species produced a new PBP of low molecular weight, while two other species produced an additional PBP of high molecular weight. With the exception of Enterococcus faecium, no difference was observed in terms of lysis or bactericidal effect when the sensitive strains and their resistant mutants were tested at ten times their respective MICs of penicillin G. With E. faecium an increased lytic and bactericidal effect was observed for the resistant mutant.