γ-Thujaplicin and β-dolabrin, the constituents of the wood of Thujopsis dolabrata SIEB. Et ZUCC. Var. hondai showed strong in vitro cytotoxic effects against the human stomach cancer cell lines KATO-III and Ehrlich’s ascites carcinoma. The cytotoxic effects of the two compounds against both tumor cell lines were clear when cell growth was measured by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) method. γ-Thujaplicin and β-dolabrin at 0.32 μg/ml inhibited cell growth of human stomach cancer KATO-III by 85 and 67%, and Ehrlich’s ascites carcinoma by 91 and 75%, respectively. There is no large difference in cytotoxicity between these compounds, but the activity of γ-thujaplicin was slightly more potent than that of β-dolabrin. On the other hand, hinokitiol acetate did not show a cytotoxic effect, suggesting that at least a part of the mechanism of the cytotoxic effect of hinokitiol-related compounds is due to metal chelation between the carbonyl group at C-1 and the hydroxyl group at C-2 in the tropolone skeleton of these molecules. The acute toxicities [50% lethal dose (LD₅₀) value: intraperitoneal injection, Van der Waedem] of γ-thujaplicin and β-dolabrin in mice were 277 mg/kg and 232 mg/kg, respectively.