Azomycin riboside (2-nitro-1-beta-D-ribofuranosylimidazole) (AR), a nucleoside analogue with the base component replaced by a 2-nitroimidazole was studied to determine its potential as a radiosensitizer. In vitro evidence showed that AR is as good as or slightly better than misonidazole (MISO) as a hypoxic cell radiosensitizer. AR was also found to kill hypoxic cells directly and this cytotoxicity was at least as great as MISO cytotoxicity. However, when tumor regrowth delay was used to assess in vivo radiosensitization, AR was found to be inferior to MISO while the LD50 host toxicity assay indicated that AR might be nearly as toxic as MISO. Unless AR proves to be less toxic than MISO or can be selectively distributed with nucleoside transport inhibitors, these preliminary observations have not shown any advantage of AR over MISO as a potential clinically useful radiosensitizer.