Tamoxifen may mediate its effect in early breast cancer in part via an oestrogen receptor (ER)-independent pathway by directly stimulating fibroblasts to produce the negative paracrine growth factor transforming growth factor (TGF)-beta. We have previously shown that secretion of this factor is induced 3-to 30-fold in human fetal fibroblasts in vitro, and by stromal fibroblasts in vivo following tamoxifen treatment of ER-positive and ER-negative breast cancer patients. Primary cultures of breast tumour fibroblasts have been exposed to tamoxifen for 48 h, and rates of secretion of TGF-beta 1 and TGF-beta 2 measured using a quantitative immunoassay. Fibroblast strains derived from malignant and benign tumours produced and secreted similar amounts of TGF-beta 1, but benign breast tumour fibroblasts secreted significantly higher levels of TGF-beta 2 compared with fibroblasts of malignant origin. Tamoxifen did not induce any consistent increase in TGF-beta secretion into the conditioned medium, but immunofluorescence analysis for the intracellular form of TGF-beta 1 revealed evidence of increased immunoreactive protein in tamoxifen-treated fibroblasts, which is localised to the nucleus. Therefore synthesis of TGF-beta 1 appears to be stimulated by tamoxifen, but increased secretion may be abrogated in vitro. Furthermore, using immunocytochemistry and transient transfection with an ER-responsive reporter construct, no ER was demonstrable in these fibroblasts supporting the proposed ER-independent paracrine pathway.