While screening the culture filtrate of microorganisms for specific inhibitors against various kinds of enzymes, we found a variety of new compounds that specifically inhibit the target enzymes1~3). Amongsuch compounds was included benarthin4'5), an inhibitor of pyroglutamyl peptidase (PGpeptidase). Wehave further continued screening for inhibitors of PG-peptidase, and discovered another new inhibitor, which we named pyrizinostatin, from culture filtrate of the microbial strain SA2289, which had been isolated from a marine soil and confirmed to belong to the genus Streptomyces. The structure of pyrizinostatin was elucidated by NMRspectral analysis and X-ray analysis, and was determined as 2,4,4a,8-tetrahydro-2,6,8-trimethyl-4a-(2-oxopropyl)pyrimido[5,4-^]- l ,2,4-triazine-3,5,7(6//)-trione (Fig. 1).