Nitric oxide (NO), the production of which is dependent on Nitric oxide synthase (NOS), has been shown to contribute to pathogeneses in various diseases. Recent investigations of NOS expression in tumor tissues indicate that NO may mediate one or more roles during the growth of human cancers. The aim of this study was to determine whether iNOS is expressed in human colon carcinoma cell lines and to determine the types of NOS isozymes in colon carcinoma cell lines with high and low metastatic potentials. METHODS: We measured the expressions of iNOS and eNOS and the formation of nitrotyrosine which indicates peroxinitrate production in highly metastatic colon cancer cell (KM1214) and lowly metastatic colon cancer cell (KM12C) by Western blots. RESULTS: The iNOS were detected in both KM1214 and KM12C by Western blot analysis. The expression of iNOS in KM1214 cells was significantly higher than in KM12C cells. The expression of iNOS was increased with lipopolysaccharide (LPS) in colon cancer cells but the rate of increase was higher in KM1214 cells than in KM12C cells. CONCLUSIONS: In human colon carcinoma cells, iNOS is expressed in cancer cells and expression of iNOS is higher in highly metastatic colon cancer cells than in lowly metastatic colon cancer cells and iNOS expression may have some role in colon cancer metastasis.