The presence of RON and its variant isoform in malignant and non-malignant human colonic tissues was examined by immunohistochemistry using paraffin-embedded sections and RT-PCR analysis followed by direct sequencing of PCR product using RNAs isolated from frozen tissues. In normal colonic mucosa, RON was uniformly expressed in crypt cells, especially in the bottom of crypta. On the other hand, the expression was distributed heterogeneously in adenomas and in colon cancer. The expression of RON was significantly related to the degree of differentiation of colon cancer and the deletion of the expression was observed in colon cancer specimens with high incidence. The RT-PCR analysis of RNA isolated from non-malignant and malignant colonic tissue revealed the presence of two RON mRNA isoforms with 432-bp and 286-bp. Direct sequencing of major product of 432-bp was revealed to be identical to that of human wild-type RON. On the other hand, major product of 286-bp was revealed to be almost identical to that of a splicing variant of RON transcript which has been found in human gastric cancer cell line, KATO-III. The results obtained in this study may indicate that both wild-type RON and its variant isoform play an important role in regulating the normal function of colonic mucosa such as differentiation and motile activity and the expression of both wild-type RON and its variant isoform could be considered to be reduced during malignancy of human colonic mucosa.