We previously isolated thymosin b15 from highly metastatic Dunning rat prostatic carcinoma cells. Immunohistochemical study of human prostate cancer specimens revealed a general correlation between Gleason grade and thymosin b15 expression, with high-grade (more malignant) tumors showing increased staining compared to low-grade tumors. To determine whether thymosin b15 may be differentially expressed in cancer cells with different metastatic potential other than in the prostatic carcinoma cells, we examined thymosin b15 mRNA levels in tumor cell lines from different species. We also examined thymosin b15 protein levels in human breast cancer samples. Thymosin b15 was upregulated in the highly metastatic mouse lung and human breast cancer cell lines in comparison to the nonmetastatic counterparts. Immunohistochemical staining showed the evidence of upregulation of thymosin b15 in malignant human breast carcinomas as compared to benign breast tumors. The expression of thymosin b1 5 was correlated with the metastatic potential of the mouse lung carcinoma and human breast carcinoma cells in addition to the prostatic carcinomas. Thymosin b15 may be a useful marker to predict metastatic potential of certain human cancers.