The antitumor nucleoside, 5-azacytidine (5-AC), is best administered clinically by prolonged intravenous infusion to minimize toxic effects. In opposition to this administration technique is facile drug decomposition in aqueous formulations giving products of unknown toxicity. Analysis of 24-h-old water solutions of 5-AC with high-pressure liquid chromatography (HPLC) indicated a threefold mixture of 5-AC, N-(formylamidino)-N'-beta-D-ribofuranosylurea (RGU-CHO), and 1-beta-D-ribofuranosyl-3-guanylurea (RGU). Preparative HPLC allowed the isolation and subsequent identification of each component in the mixture, including RGU-CHO which until now has not been available for chemical and biological study. It was shown that RGU-CHO in water solution readily equilibrates to 5-AC and more slowly deformylates to give RGU irreversibly. The latter hydrolysis produce exhibited no pronounced toxicity when tested either in vitro or in vivo. Although RGU-CHO showed considerable antitumor activity against murine L1210 leukemia, hydrolysis studies indicated that all of the observed activity could be attributed to 5-AC formed by in vivo equilibration from RGU-CHO. Moreover, RGU-CHO seemed to impart to test animals a toxicity which was no greater than that anticipated from its ability to generate 5-AC.