Overexpression of thymosin ß-4 has been linked to malignant progression but the localization of this polypeptide within tumor is incompletely known. We therefore examined breast cancers for thymosin ß-4 using immunofluorescence. Reactive cells were identified with monoclonal cell marker antibodies. A very heterogeneous staining pattern for thymosin ß-4 was observed. Thus, while leukocytes and macrophages showed intense reactivity for this polypeptide, cancer cells, and endothelial cells showed a much more variable reactivity. A similar heterogeneous staining was observed also in colorectal carcinomas. The degree of staining of breast cancer cells for thymosin ß-4 correlated neither to histological grade nor to endothelial cell staining. However, therewas a tendency toward correlation (P = 0.07) between staining of endothelial cells and histological grade. Treatment of cultured breast cancer cells (SK-BR-3) with 1-4 µg thymosin ß-4/mL significantly increased cell numbers, as determined by MTT-assays. These data reveal an unexpected cellular heterogeneity of thymosin ß-4 expression in breast and colonic carcinomas and suggest that local release of this polypeptide in the tumor microenvironment may modulate tumor behavior.