Summary Tumor inhibitory activity of 5-fluorouridine-5′-monophosphate (FURP) and 5 - fluoro − 2′ - deoxyuridine − 5′ - monophosphate (FUDRP) have been studied in mice bearing Sarcoma-180, Ehrlich ascites carcinoma, and L-1210 leukemia. Comparisons have been made with equimolar equivalents of corresponding nucleosides, 5-fluorouridine (FUR) and 5 − fluoro − 2′ - deoxyuridine (FUDR). FUDR and FUDRP were inhibitors of S-180 of not strikingly different potency. FUR and FURP were inactive against this tumor. However, FURP at lower dose was as active as FUDRP against Ehrlich ascites carcinoma, producing about 30% 100 day survivors. FUR was extremely toxic. In L-1210 leukemia the compounds were of moderate and equivalent activity.