A novel guanosine analog, aminonucleoside antibiotics (2AG), produced by Enterobacter sp., was identified as 9-(2'- ext{amino-}2'- ext{deoxy-}eta- ext{p-ribofuranosyl})$ -guanine (2'-amino-2'deoxyguanosine) by proton magnetic resonance and carbon-13 nuclear magnetic resonance spectra of 2AG and acetylated 2AG, and also by the comparison of the aminosugar obtained by hydrolysis of 2AG with 2-amino-2-deoxypentoses appeared in the literatures. This is the first report on the occurrence of 2-amino-2-deoxy-p-ribose in nature. The isolation and preliminary structure determination of a novel aminonucleoside produced by Enterobacter sp. were reported in the previous paper.3 The aminonucleoside (named 2AG) showed antibacterial activity against only some Escherichia coli and antitumor activity. Many nucleoside antibiotics have been discovered from microorganisms, but only puromycin and 3'-amino-3'-deoxyadenosine were known as aminoribosyl-nucleoside.4) It was suggested that 2AG consisted of guanine and 2-amino-2-deoxypentose. The occurrence of 2-amino-2deoxypentose in nature has not been reported. Therefore, we undertook a detailed study on the structure of 2AG. This paper describes the structure elucidation of 2AG.