The antitumor activity of 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl) cytosine (ECyd) and 1-(3-C-ethynyl-beta-D-ribo-pentofuranosyl) uracil (EUrd), designed as a potential multifunctional antitumor nucleoside to inhibit RNA and DNA syntheses, was examined. ECyd and EUrd inhibited the growth of 47 kinds of cultured human cells in vitro and also showed strong antitumor effects on 15 human solid cancers xenografted into nude mice at a dose of 0.25 mg/kg (ECyd) or 2 mg/kg (EUrd) by intravenous administration for 10 consecutive days. The in vitro cytotoxic effect of ECyd and EUrd was prevented dose dependently by cytidine and uridine, suggesting that ECyd and EUrd may require phosphorylation by uridine/cytidine kinase for antitumor activity. ECyd and EUrd strongly inhibited RNA synthesis and slightly inhibited DNA synthesis. ECyd and EUrd have shown potent antitumor activity against human experimental solid type tumors with minimal toxic effects in vivo, suggesting that ECyd and EUrd is a promising agent with a unique mechanism of action for the treatment of cancer.