The cytotoxic activities of Cinnamomum cassia (Blume) bark-derived materials toward six human HeLa epithelioid cervix, A549 lung, SK-OV-3 ovarian, SK-MEL-2 melanoma, XF-498 central nerve system, and HCT-15 colon tumor cell lines were evaluated by using sulforhodamine B assay and compared to those of the anticancer agents, cisplatin and mitomycin C. The biologically active constituent of the Cinnamomum bark was characterized as trans­cinnamaldehyde by spectroscopic analysis. The cytotoxic activity of cinnamaldehyde against HeLa, SK-MEL-2, and HCT -15 cell lines was comparable to that of cisplatin and mitomycin C. The compound showed lower activity against A549, SK-OV-3, and XF-498 cell lines than the anticancer agents. Eugenol exhibited moderate activity against SK-OV­3, XF-498, and HCT-15 tumor cells, and trans-cinnamic acid, cinnamyl alcohol, $alpha-pinene,;and;eta-pinene$ showed little or no activity against model tumor cells. Cinnamaldehyde was not mutagenic against four strains (TA 98, TA 100, TA 1535, and TA 1537) of Salmonella typhimurium (Castel and Chalm). These results indicate at least one pharmacological action of C. cassia.