This study was to evaluate the cytotoxic effect of three root canal disinfectants (formocresol, camphorated phenol and eugenol) and ten root canal sealers(Cavitec, Hypo-cal, Vitapex, AH26, Canals, Mynol, $N_2$, $N_2$-Medical, Z. O. E. and Calvital) in vitro. The experiments were performed in four differrent modes. In the first and second experiment, the "long-distance" cytotoxicity of three root canal disinfectants were tested on L cells. In the third exeriment, ten root canal sealers were tested for cytotoxicity by means of the tissue culture-agar overlay method immediately, 4 and 24 hours after the experiment. In the fourth experiment, the study with radioactively labeled L cells were employed to determine the relative cytotoxicity of ten root canal sealers. The results were as follows; 1. Every vapors from disinfectants showed more or less cytotoxicity. Of the three disinfectats, formocresol appeared to be the highest cytotoxic effect and camphorated phenol was the lowest. 2. Root canal sealers tested in tbis study showed cytotoxicity at every stage of time intervals. 3. The highest cytotoxic effect was freshly mixed $N_2$ meaical and $N_2$ also revealed the highest cytotoxic effect after 4 or 24 hours among these materials. Vitapex was found the lowest cytotoxic effect at all experimental stage. 4. Root canal sealers except N2 and Mynol showed cytotoxic effect were decreased cytotoxicity according to the time elapsed.