Carcinogenicity of di(2-ethylhexyl)phthalate(DEHP) to the mose forestomach and its inhibitor activity for the initiation of Benzo[a]pyrene(BP)-induced mouse forestomach neoplasia were studied on the mouse forestomach carcinogenesis regimen. One hundred female ICR mice(6~7 weeks of age) were hosed in a poly-carbonate cage (4 mice/cage) in a humidity- and temperature-controlled room subjected to a semipurified diet for a week. Mice were divided into 4 treatment groups (25 mice/treatment): Basal diet, DEHP, BP, and BP+DEHP. On Monday and wednesday, 0.1ML DEHP mixed with 0.1ml olive oil (for DEHP and DEHP+BP treatment groups) or 0.1ml saline+0.1ml olive oil (for basal diet group) was intubated, p.o., and on Friday, 2mg BP dissolved in 0.2ml olive oil (for BP and BP+DEHP treatment groups) was intubated, p.o. This cycle was repeated for 4 weeks. Beginning with the first intubation of BP an continuing thereafter, body weight and food intake were recorded once and twice weekly, respectively. All surviving mice were sacrificed 22 weeks after the first dose of BP intubation and countered forestomach tumor. No tumor was formed by DEHP treatment. 5.75 tumors per mouse was formed by BP treatment, whereas its number was reduced to 4.53 by BP+DEHP treatment. Similar results were seen in the tumor incidence. Body weight gain was not affected by DEHP treatment, when compared to that b basal diet treatment. The body weight was significantly reduced by BP treatment, but its reduction was recovered to the level of the basal diet group by BP+DEHP treatment. No significant difference was seen in food intake among all treatment groups. These results indicate that DEHP lacks carcinogenic activity to the mose forestomach and rather inhibits the initiation of BP-induced mose forestomach neoplasia.