Adecypenol, which exhibits potent inhibitory activity against calf intestinal adenosine deaminase (EC 3.5.4.4), was isolated from the cultured broth of Streptomyces sp. OM-3223. Adecypenol was classified as a semi-tight binding inhibitor. The Ki value against calf intestinal adenosine deaminase was 4.7 X 10(-9) M. No acute toxicity of adecypenol was observed at 100 mg/kg in mice. Adecypenol exhibited no antimicrobial activity against various bacteria and fungi at the concentration of 1.0 mg/ml.