Proliferation and death were measured in cultures of mouse neuroblastoma N18TG-2 and rat glioma C6BU-1 cells during treatment with up to 100 uM retinoidal 4-ylidenebutenolides (RYB 1-3), whose chemical structures are already reported (Heterocycles, 19, 1385, 1982). The number of viable cells were measured after 2-days incubation with various concentrations of the compounds, among which RYB-3 (figure) was the most potent one to destroy the cells. IC50 of RYB-3 was about 0.5 uM for both types of cells, which was 80-fold more susceptible than that of retinoic acid. The degenerative changes of RYB-3 on C6BU-1 cells were irreversible even when the cells were exposed for 2 hrs. Tumor weights of N18TG-2 cells inoculated subcutaneously on the backs of A/J mice were 30-40% less than those of untreated controls after 14 days of single daily i.p. injections of RYB-3 at the dose of 100 mg/kg of the body weight. The results indicate that RYB-3 is cytotoxic on the tumor cells originating from the nervous system and has a inhibitory effect on neuroblastoma tumor growth in mice.