AIM:To explore the inhibition of conjugated linoleic acidisomers in different purity (75% purity c9, t11-, 98% purityc9,t11- and 98% purity t10, c12-CLA) on the formation offorestomach neoplasm and cheopreventive mechanisms. METHODS: Forestomach neoplasm model induced by B(a)P in KunMing mice was established. The numbers of tumorand diameter of each tumor in forestomach were counted;the mice plasma malondialdehyde (MDA) were measuredby TBARS assay; TUNEL assay was used to analyze theapoptosis in forestomach neoplasia and the expression ofMEK-1, ERK-1, MKP-1 protein in forestomach neoplasm werestudied by Western Blotting assay.RESULTS: The incidence of neoplasm in B(a)Pgroup, 75%purity c9, t11-CLA group, 98% purity c9,t11-CLA groupand 98% purity t10, c12-CLA group was 100%, 75.0 %(P>0.05), 69.2%(P<0.05) and 53.8%(P<0.05) respectivelyand the effect of two CLA isomers in 98% purity onforestomach neoplasia was significant; CLA showed noinfluence on the average tumor numbers in tumor-bearingmouse, but significantly decreased the tumor size, the tumor average diameter of mice in 75% purity c9,t11-CLA group,98% purity c9,t11-CLA group and 98% purity t10, c12-CLAgroup was 0.157