Biological activity of neutral, acidic, and basic compounds bearing dibenzo-p-dioxin (D) skeleton as well as phenoxathiin (OSD) , and thianthrene (SSD) was examined in vivo.
 On Tumor-promotion experiments indexed by the expression of Epstein-Barr virus early antigen caused by a tumor promotor TPA,most of neutral D derivatives indicated abti-tumor-promoting activity. Especially, 2,7-dichloro-D has a remarkable activity. At 500 times concentration relative to TPA,it inhabited tumor promotion by 80% without giving injuries on Raji cells even at 32 nano moles.
 Acidic D derivatives such as D-1,6- and 2,7-dicarboxylic acid, their methyl esters, 2,7-disulfonic acid, 2,7-dimethyl-3,8-disulfonic cid, 1,6-dimethoxy-3,8-dicarboxylic acid, and its methyl ester indicated remarkable anti-tumor-promoting activity. Although D itself has no significant activity, neutral 1,6- and 2,7-dimethyl-D,1,6-dimethoxy-3,8-dimethyl-D,1,6-dimethoxy-D-3,8-dialde-hyde, 2,7-dimethoxy1-3,8-dinitro-D,as well as basic 2,7-dimethyl-3,8-diamino-D and 1,6-dimethoxy-3,8-diethylamino-D have anti-tumor-promoting activity. Among these compounds, the most active methyl D-2,7-dicarboxylate inhibits tumor promotion by 80% at 100 times concentration relative to TPA.
 At 100 times concentration relative to TPA,trilobine and isotrilobine, bisbenzyl-isoquinoline alkaloids of Cocculus trilobus comprising a dibenzo-p-dioxin skeleton in the molecule, inhibit the tumor promotion by 60 and 50%, respectively. The viability of the Raji cells were 40 and 50%, respectively.
 At 1000 times concentration relative to TPA,SSD and OSD completely inhibit tumor promotion. At this concentration, both compounds did not show toxicity to Raji cells.