A series of 4-keto unsaturated C-glycosides was synthesized from 0-acetyl C-glycosides. These compounds and some parent 2-keto unsaturated C-glycosides were tested for cytotoxic activity against LFCl2A cells (Rat hepatocarcinoma cells). C-Glycosides with an acyclic aglycone have not been found cyctotoxic, whereas 1-methyl cyclohexenyl and methylene cyclohexenyl C-glycosides possessed IC50 values of 9-0.03-mu-M with 11 being the most potent.