Mallotus japonicus MUELL. ARG. (Euphorbiaceae) is a deciduous tree that sprouts red-colored buds and is widely distributed in Japan. The leaves of the tree are used as a treatment for boils. We have examined cytotoxic constituents in pericarps of the tree. In a investigation of the CHCl_3-soluble fraction, which was found to show significant cytotoxic activity in the KB cell culture system, eight new phloroglucinol derivatives named mallotolerin (5), isomallotolerin (6), mallotochromene (7), butyrylmallotochromene (8), isobutyrylmallotochromene (9), mallotophenone (10), mallotochromanol (11) and isomallotochromanol (12) were isolated together with five known compounds, 1-4 and 13. Their structures have been characterized by their respective chemical and spectral data. The unambiguous assignments of ^<13>C-NMR spectra of compounds 7-10 and 12 were performed by means of 2D NMR of long rang ^1H-^<13>C shift correlation. These compounds were assessed for growth inhibiting activity against KB, human larinx (Hep-2) and lung (PC-13) carcinoma cells as well as mouse B-16 melanoma, leukemias P-388 and L-5178Y cells. Most of these derivatives were prove to be significantly cytotoxic in culture. One of them 3-(3,3-dimethylally1)-5-(3-acety1-2,4-dihydroxy-5-methy1-6-methoxybenzy1)-phroracetophenone (1) showed an excellent cytotoxicity against all target tumor cells and marked prolongation of life-span in mice bearing L-5178Y. Analysis of the DNA histogram obtained by flow cytometry showed that 1 inhibited the cell cycle progression.