We identified a thymosin-β4 gene overexpression in malignant mouse fibrosarcoma cells (QRsP-30) that were derived from clonal weakly tumorigenic and nonmetastatic QR-32 cells by using a differential display method. Thymosin-β4 is known as a 4.9-kd polypeptide that interacts with G-actin and functions as a major actin-sequestering protein in cells. All of the six malignant fibrosarcoma cell lines that have been independently converted from QR-32 cells expressed high levels of thymosin-β4 mRNA and its expression in tumor cells was correlated with tumorigenicity and metastatic potential. Up-regulation of thymosin-β4 in QR-32 cells (32-S) transfected with sense thymosin-β4 cDNA converted the cells to develop tumors and formed numerous lung metastases in syngeneic C57BL/6 mice. In contrast, antisense thymosin-β4 cDNA-transfected QRsP-30 (30-AS) cells reduced thymosin-β4 expression, and significantly lost tumor formation and metastases to distant organs. Vector-alone transfected cells (32-V or 30-V cells) behaved like their parental cells. We observed that tumor cell motility, cell shape, and F-actin organization is regulated in proportion to the level of thymosin-β4 expression. These findings indicate that thymosin-β4 molecule regulates fibrosarcoma cell tumorigenicity and metastasis through actin-based cytoskeletal organization.