Meticillin is an antibiotic primarily used to treat infections caused by Gram-positive bacteria, especially those resistant to other penicillins. It is effective against Staphylococcus aureus, including strains that produce penicillinase, an enzyme that inactivates some antibiotics. Meticillin works by inhibiting bacterial cell wall synthesis, leading to cell lysis and death.

Due to its instability in acidic environments, meticillin is not administered orally. It is given by injection and is used mainly in hospital settings for serious infections such as septicemia, endocarditis, and bone or joint infections caused by susceptible organisms.

Because of the risk of allergic reactions and the development of methicillin-resistant Staphylococcus aureus (MRSA), its clinical use has declined. It has largely been replaced by more stable and effective antibiotics. However, meticillin played a key historical role in the development of penicillinase-resistant penicillins and in understanding antibiotic resistance.