In the course of our screening for enzyme inhibitors from microorganisms, a new adenosine deaminase inhibitor, which was named adecypenol, was isolated from the cultured broth of Streptomyces sp. OM-3223. The structure was determined based on spectral data. It has the homopurine aglycone, identical with that of the known adenosine deaminase inhibitors coformycin, 2'-deoxycoformycin and adechlorin, which attaches to the C-3 position of 5-(hydroxymethyl)-4-cyclopentene-l,2-diol found in neplanocin A. This paper describes the isolation, physico-chemical properties and structure determination of adecypenol.