Methicillin is the leader in penicillin M.
Penicillins in group M are bactericidal antibiotics of the beta lactam family. Bacteria have developed penicillinases to make these antibiotics ineffective. In contrast to penicillins in group "G" and "V", penicillins in group M are resistant to penicillinases of staphylococci. This explains the relative efficacy and the lagging of resistance against this group of penicillin compared to other groups.