As reported in previous papers, chemical screening of microbial culture filtrates gives new compounds of structural interest. In this paper, we report the isolation and structural studies of argvalin (1) which was discovered by searching for compounds with positive Wood and diacetyl color-reactions. The culture filtrate (10 liters, pH 6.7) of a strain KG62-AG1 (closely related to Streptomyces filipinensis) was extracted with n-butanol at pH 8, and the extract was purified by successive column chromatography on acidic alumina (with methanol), cellulose [with n-butanol-ethanol-water (6:1:2)] and Dowex 1x2 (OH form, with water), followed by neutralization with hydrochloric acid and recrystallization from isopropyl alcohol and ethyl acetate to give colorless needles of argvalin monohydrochloride. Argvalin is a basic substance positive to Wood, diacetyl and Sakaguchi reagents but negative to ninhydrin and Ehrlich reagents. Structural elucidation was carried out using IR, UV, Mass and NMR spectra, chemical conversions (e.g., conversion to the corresponding dioxopiperazine, hydrogenation, hydrolysis to valine and arginine), and synthesis of a reference compound 3S:6S-6-(3-guanidinopropyl)-3-isopropyl-2,5-dioxopiperazine whose IR and NMR data were identical to those of the derivative from argvalin. From the above results, the structure of argvalin was determined to be 6-(3-guanidinopropyl)-3-isopropyl-2(1H)-pyrazinone (1).