Objective: Eugenol, hydroquinone, benzoquinone, phtalaldehyde are used in the dentistry and also as the antiseptic or the medical agents exclusively used as cosmetics. To use these highly reactive compounds safely in the dentistry, it is necessary to investigate the cytotoxicity against both normal and tumor cells at the same time. Eugenol and hydroquinone have previously been reported to induce the apoptosis in human promyelocytic leukemia cell line. However, no study has been reported about the cytotoxicity of these compounds against normal cells, in conjunction with that against tumor cells. Therefore, the cytotoxicity of these four compounds against human three oral squamous cell carcinoma cell lines vs. human oral normal cell [gingival fibroblast, pulp cell, periodontal ligament fibroblast], and the type of cell death induced were investigated in the present study. Since many toxicants show biphasic effects on the growth, that is growth stimulating (hormetic) and inhibitory effects at lower and higher concentrations, respectively, the possible hormetic effects of these compounds were also investigated. Methods: Viable cell number was determined by MTT method. The 50% cytotoxic concentration (CC50) was determined from the dose-response curve. The relative cytotoxicity against tumor cells vs. normal cells (RC value) was determined by the ratio of mean CC50 value for normal cells to that for tumor cells. DNA fragmentation was monitored by 2 % agarose gel electrophoresis. Caspase-3 activation was monitored by the cleavage of the substrate (DEVD-p-nitroanilide,). Results: All four compounds showed comparable cytotoxicity against both normal and tumor cells (RC=0.8-1.0), without inducing any hormetic effects at lower concentrations during 0-48 hours. These compounds did not induce the caspase-3 activation nor internucleosomal DNA fragmentation, suggesting the induction of cell death other than apoptosis. Conclusion: The lower RS value suggests the necessity of cautionary use of these compounds in the oral cavity.