The chemical structure of a new dipeptide antibiotic, TAN-1057 A, isolated from the broth filtrate of Ffexibacter sp. PK-74 was determined to be (3'S, SS)-5-[N-methyl-N-(3'-aminod' guanidinohexanoyl)amino]-5,6-dihydro-2-ureido-4( lH)-pyrimidone. The antibiotic was specifically active against staphylococcus spectes including methicillin-resistant strains.