Cadeguomycin, a new nucleoside analog antibiotic, has been purified as colorless needle crystals by recycling preparative HPLC. The antibiotic, C12H14O7N4, mp 231 approximately 239 degrees C (dec.), FD-MS: m/z 326 (M+); is a weakly acidic substance, showing UV gamma H2Omax (epsilon) 232 (19677), 272 (6881) and 298 nm (7607), and IR nu KBrmax 1650 (C = O) and 3420 (NH or OH) cm-1. The UV spectrum is similar to other pyrrolo[2,3-d]pyrimidines. The structure of cadeguomycin, 2-amino-3,4-dihydro-4-oxo-7-beta-D-ribofuranosyl-7H-pyrrolo[2,3-d]pyrimidine-5- carboxylic acid, has been elucidated by 1H NMR and 13C NMR in comparison with other pyrrolo[2,3-d]pyrimidines and their nucleosides.