Hinokitiol (I) and tropolone (II) showed characteristic cytotoxic effects in vitro on five kinds of human and murine cell lines and blastic lymphocytes from mouse splenocytes. The cytotoxic effect of I on the growth of murine and human tumor cell lines, including RL♂-1, MH134, HL60, K562 and KATO-III was definite when examined by thymidine incorporation into DNA and its 50% inhibitory concentration (IC₅₀) on all cells was 0.3-0.6 μg/ml. Compound II also showed comparable cytotoxic effects on these cell lines, indicating a little lower activity when compared to I. Furthermore, I and II also completely suppressed the [³H]thymidine ([³H]TdR) incorporation of mitogen-induced blastic lymphocytes. The suppressive activity on mouse lymphocyte proliferative response to concanavaline-A was also found with both compounds at a low concentration of 0.32 μg/ml. As compound I is known to be of fairly low toxicity (LD₅₀ : 453+24 mg/kg in mice), the antitumor and immuno-suppressive effect of hinokitiol (I) should be further investigated.