NEW MATERIAL:A compound shown by the formula (X and Y are OH or acetoxy) and its salt.

EXAMPLE: PF1057A.

USE: The titled substance has β-lactamase inhibiting activity, and the use of the substance and a β-lactam-based antibiotic (e.g., benzylpenicillin, cephlexin) increases synergistically the antimicrobial action of the antibiotic.

PROCESS: A microorganism [e.g., Chaetomella cinnamomea PF-1057 (FERM-P 6491)] belonging to the genus Chaetomella, capable of producing PF1057, is cultivated under aerobic conditions in a medium, preferably liquid culture is carried out at about 20W40°C, preferably at 25W32°C for about 72hrW10 days. After the cultivation is over, the culture is purified and separated by filtration, centrifugation, column chromatography using various kinds of ion exchange resins, to give the desired PF1057.