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AIM: To investigate the effect of ethylene diamine tetraacetic acid (EDTA) on proliferation of rat colonic cells.METHODS: EDTA was administered into Wistar rats,carcinogenesis induced by 1,2-dimethylhydrazine (DMH) in rats was studied with immunohistochemistry.RESULTS: Marked regional differences in cell proliferation were found in all groups. In EDTA-treated animals, total labelling indexes in both proximal (10.00±0.44 VS7.20±-0.45)and distal (11.05±0.45 vs8.65±0.34) colon and proliferative zone size (21.67±1.13 vs 16.75±1.45, 27.73±1.46 vs 21.74±1.07) were significantly higher than that in normal controls (P＜0.05) and lower than that in DMH group (10.00±0.44 vs 11.54±0.45, 11.05±0.45 vs 13.13±0.46,21.67±1.13 vs 35.52±1.58, 27.73±1.46 vs 39.61±1.32,P＜0.05). Cumulative frequency distributions showed a shift of the EDTA distal curve to the right (P＜0.05) while the EDTA proximal curve did not change compared to normal controls. Despite the changes of proliferative parameters,tumours did not develop in EDTA treated animals.CONCLUSION: Hyperproliferation appears to be more easily induced by EDTA in distal colon than in proximal colon.Hyperproliferation may need to exceed a threshold to develop colonic tumours. EDTA may work as a co-factor in colonic tumorigenesis.