Thai bitter gourd fruits (Momordica charantia Linn., TBG) has been previously demonstrated to possessphase II detoxificating enzymes inducing properties, as well as the ability to reduce phase I carcinogen activatingenzyme activity in rat liver. In addition, it was partially inhibited 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary gland carcinogenesis in female Sprague-Dawley rats. In this study, we therefore examined theanticlastogenic and anticarcinogenic effect of TBG against clastogens, cyclophosphamide (CYP) and DMBA, inmice using the in vivo erythrocyte micronucleus assay and azoxymethane (AOM)-induced colon carcinogenesisin rats, respectively. For anticlastogenicity test, male mice were fed with modified AIN-76 diets containing6.25% and 12.5% of ground freeze-dried TBG for 2 weeks prior to administration of clastogens till the end ofexperiment. Blood samples were collected and counted for reticulocytes by using the fluorescent microscope. Foranticarcinogeicity test, male Wistar rats were fed with modified AIN-76 diets containing 5% and 10% groundfreeze-dried TBG for 2 weeks prior to, during and 1 week after the completion of AOM administration (15 mg/kgonce a week for 2 weeks). It was found that TBG at 6.25% resulted in a significant reduction in micronucleatedperipheral reticulocytes (MNRETs) induced by only CYP. Study on anticarcinogenic potential demonstratedthat rats fed with TBG diets at the concentration tested developed significantly higher incidence as well as themultiplicities of colon tumors than the control group. These results demonstrated that Thai bitter gourd fruitspossesses anticlastogenic potential against clastogen in the mouse. Interestingly, it had no preventive potentialagainst AOM-induced colon carcinogenesis in rat, rather increasing the incidence of colonic neoplasm whengiving during the initiation stage.