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We have previously shown that a diet containing a mixture of conjugated linoleic acid (CLA) isomers reducesthe incidence of colon tumors in rats treated with 1,2-dimethylhydrazine (DMH). The present study examined which of thetwo main CLA isomers, trans-10,cis-12 CLA (t10c12) or cis-9,trans-11 CLA (c9t11), decreases colon tumor numbers and themechanisms for this effect. Six-week-old, male Sprague-Dawley rats were intramuscularly injected with 15 mg/kg of DMHtwice per week for 6 weeks and fed a control diet, 1% t10c12, or 1% c9t11 for 30 weeks. The experimental diets were initi-ated simultaneously with DMH injection. The tumor numbers were decreased and the apoptotic index was significantly in-creased in the colonic mucosa of the t10c12 and c9t11 groups, when the results were compared with those of the controlgroup. The protein levels of Bcl-2 and cyclooxygenase-2 were significantly decreased, but Bax levels were increased in bothof the CLA isomer groups. The thromboxane B2 levels in colonic mucosa were substantially lower in the two CLA isomergroups than in the control group. However, there was no difference in these parameters between the CLA isomer groups. Wehave demonstrated that diets containing 1% t10c12 and c9t11 were equally effective in reducing tumor numbers and induc-ing apoptosis in the colonic mucosa of rats treated with DMH. These results indicate that Bcl-2 family protein levels are as-sociated with CLA-induced apoptosis in the colonic mucosa of DMH-treated rats.