A series of piperazine-2,5-diones in which both amino acid constituents are hydroxamic acid residues has been isolated from Aspergillus terreus Thom. The metabolites were identified by i.r., 'H n.m.r., and mass spectroscopy as (3S,6S)-l,4-dihydroxy-3,6-bis[(l R)-1-methylpropyl]piperazine-2,5-dione (terramide A) (1), (3S,6S)-1,4-dihydroxy-3-isopropyl-6-[(1R)-1-methylpropyl]piperazine-2,5-dione (terramide B) (2), and (3S,6S)-1,4-dihydroxy-3,6-bisisopropylpiperazine-2,5-dione (terramide C) (3). Separation of the three components in the mixture was achieved by reverse-phase high pressure liquid chromatography (h.p.1.c.). The S configuration of the chiral centres was established by reduction and hydrolysis of the terramide mixture to the component amino acids and analysis by g.1.c. using a chiral column.