The 6-alkenyluracil nucleosides (7a-e) and bases (14a-i) were synthesized and evaluated for their ability to inhibit the growth of mouse leukemia L1210 cells in vitro. While several of the free bases are cytotoxic, with IC50 values in the 1 - 5 mu M range, the corresponding beta-D-ribofuranosyl nucleosides are not significantly active.