Formycin, a C-glycoside analogue of adenosine, is rapidly converted to phosphorylated derivatives by Ehrlich ascites tumor cells in vitro. In the presence of an adequate energy supply, the 5′-triphosphate was the major phosphorylated metabolite of formycin; under some conditions, high levels of formycin 5-monophosphate and 5′-diphosphate also accumulated in the cells. In most experiments, the concentration for formycin phosphates approached that of the adenosine phosphates. Formycin was also extensively deaminated to the corresponding inosine analogue, formycin B; the ratio of deamination to phosphorylation was approximately 5. When formycin was administered to mice bearing the Ehrlich ascites tumor, formycin phosphates were detected in extracts of the tumor cells; again, formycin 5′-triphosphate was the major nucleotide metabolite.