Corilagin and tellimagrandin I are polyphenols isolated from the extract of Arctostaphylos uvaursi and Rosa canina L. (rose red), respectively. We have reported that corilagin and tellimagrandin I remarkably reduced the minimum inhibitory concentration (MIC) of beta-lactams in methicillin-resistant Staphylococcus aureus(MRSA). In this study, we investigated the effect of corilagin and tellimagrandin I on the penicillin binding protein 2 '(2a) (PBP2 '(PBP2a)) which mainly confers the resistance to beta-lactam antibiotics in MRSA. These compounds when added to the culture medium were found to decrease production of the PBP2 '(PBP2a) slightly. Using BOCILLIN FL, a fluorescent-labeled benzyl penicillin, we found that PBP2 '(PBP2a) in MRSA cells that were grown in medium containing corilagin or tellimagrandin I almost completely lost the ability to bind BOCILLIN FL. The binding activity of PBP2 and PBP3 were also reduced to some extent by these compounds. These results indicate that inactivation of PBPs, especially of PBP2 '(PBP2a), by corilagin or tellimagrandin I is the major reason for the remarkable reduction in the resistance level of beta-lactams in MRSA. Corilagin or tellimagrandin I suppressed the activity of beta-lactamase to some extent.