A new type of antineoplastic nucleoside, 2'-deoxy-2'-methylidenecytidine (DMDC) has been synthesized from the corresponding 2'-keto pyrimidine nucleosides 3 and 8 by the Wittig reaction. During the course of the reaction, we found that an intermediate betaine could pick a proton from the excess triphenylphosphonium bromide to form the 2'-phosphonium salts 5 and 10, which could be further converted into the 2'-deoxy-2'-methylidene nucleosides 4 and 9 by treatment with sodium hydride. Various 5-substituted DMDC derivatives 19a-e,h and their uracil congeners 16a-h were also synthesized from the corresponding 5-substituted uridines 12a-f,h. Among them, DMDC as well as 2'-deoxy-2'-methylidene-5-fluorocytidine (19a) showed potent antileukemic activity against murine L1210 cells in culture. The activity of DMDC and 19a toward various human tumor cells in culture compared with 1-beta-D-arabinofuranosylcytosine and 5-fluorouracil was also examined. In vivo antitumor activity of DMDC against L1210 was also described.