MRSA(methicillin-resistant Staphylococcus aureus) has a unique novel protein, Mec A protein, which is responsible for methicillin-resistance and has a characteristic inducible-synthesis system by β-lactam antibiotics. Previously, DNA sequences of 4 kb HindIII fragment containing mec A gene were determined, anaylsed. In this fragment, two open reading frame (ORF) had been reported; one is mec A structural gene, which encoded Mec A protein (670 amino acids residues), and another one which encodes 169 amino acids residues, what we called, is mec B gene which located on 100 base pairs (bp) upstream of the mec A structural gene, which also may encode for some regulatory protein, but not yet identified mec B gene product. And the functions of mec B gene are poorly understood. This present work has shown that the inverted-directed ORF (mec B) for the mec A structural gene encoded the protein of about 18,000 daltons using in vitro translation system.