Cycloserine (4-amino-3-isoxazolidinone) is an amino acid derivative with an unusual structure. It is a colorless solid that is produced by some bacteria. It has emerged as a drug sold under the brand name Seromycin. It is an antibiotic effective against Mycobacterium tuberculosis. It is on the World Health Organization's List of Essential Medicines, the most important medications needed in a basic health system.