We constructed a "non-metastatic cell (SW837)--metastatic cell (PMCO1)" subtraction library and identified one cDNA that was strongly expressed in SW837 but weakly expressed in PMCO1. The nucleotide sequence of the cDNA revealed that it encoded thymosin beta-4. Four non-metastatic cell lines, which produced no experimental liver metastasis in nude mice, showed high expression of thymosin beta-4. However three of four metastatic cell lines showed weak expression of thymosin beta-4. Among surgical materials, thymosin beta-4 expression was high in tumors without metastasis in comparison with non-tumorous mucosa, but one case with liver metastasis showed decreased expression in both the primary and metastatic tumors. We suspect that down-regulation of thymosin beta-4 expression is correlated with the metastatic capacity of colorectal carcinomas.