The expression of calretinin, a calcium-binding protein, has been studied in a series of 82 human colorectal adenocarcinomas. In 22.5% of the cases, part of the tumor cells were calretinin-positive, whereas the cells of the normal and paratumoral mucosa were always negative. Two types of cells from the tumoral mass reacted positively and selectively with calretinin-antisera: the tumor cells and giant fibroblasts. The neurons of enteric ganglia and reactive mesothelial cells also reacted positively to the same antibody. The results obtained by immunochemistry have been confirmed by Western blot analysis and in situ hybridization for calretinin mRNA. There is a correlation between the expression of calretinin and the degree of differentiation of the tumor. Well-differentiated tumors express calretinin in only 5% of the cases, whereas this percentage is 20% for moderately differentiated tumors and 66.6% for poorly differentiated or undifferentiated tumors. We conclude that calretinin is expressed by most undifferentiated colorectal adenocarcinomas, but only by a limited number of cells in well-differentiated tumors. The degree of its expression coincides also with additional signs of malignancy, such as an increase in the number of metastases in the regional lymph nodes and in other organs.