In order to study the antitumoral effect of Selaginella tamariscina extracts, the cytotoxicities to human histiocytic leukemia cells (U937) and lymphocyte were measured by MTT method. The water extract of Selaginella tamariscina was partitioned into chloroform (CHCl₃), ethylacetate (EtAc), n-butanol (BuOH) and water (H₂O), successively. CHCl₃, EtAc and BuOH fractions of Selaginella tamariscina showed the cytotoxicity to the U937 cells but they had no effect on the cytotoxicity of lymphocyte under the same conditions. The tumor-specific cytotoxicity of Selaginella tamariscina fractions might have been attributed to their genotoxic effect on actively proliferating cells. The expression of p53 tumor suppressor gene was then evaluated by northern blotting. The increased expression of p53 was induced by Selaginella ramariscina fraction V but no expression of p53 was induced by CHCl₃, EtAc, and BuOH fractions of Selaginella tamariscina. These results suggested that the increased expression of p53 induced by Selaginella tamariscina water extract (fraction V) should be required for the cytotoxicity on U937 and the other fractions of Selaginella tamariscina mediated the U937 disruption.