It was shown that CEACAM5 can mediate cell-cell adhesion through homotypic and heterotypic interactions; however, its role in the expression of the malignant phenotype remains obscure. To study whether the formation of both primary tumors and metastases is directly related to the presence or absence of CEACAM5, we applied the antisense RNA strategy. By transfecting human CX-1.1 colon carcinoma cells with CEACAM5 antisense-expressing vector or with the vector itself, cell variants with a highly decreased expression of CEACAM5 were obtained. Profound differences in proliferative abilities among parental and obtained subclones of CX-1.1 cells were revealed when cells were implanted subcutaneously into nude mice. In contrast to their highly tumorigenic parental CX-1.1 cells (with high expression of membrane-bound and secreted CEACAM5), two subclones (3E and AS6Q) with substantially decreased expression of membrane-bound and secreted CEA showed a considerably diminished growth rate. Even more striking results were obtained with AS8Q cells, producing a residual amount of this glycoprotein. However, 3B cells (producing a large amount of secreted CEACAM5) did not differ significantly in their tumorigenic properties from CX-1.1 cells. Our experiments performed in nu/nu mice suggest that CEACAM5 supports the growth of primary tumors, but is not involved in the formation of metastases by colon cancer cells.