Fas (CD95/Apo-1) is a cell membrane receptor that upon binding by its ligand (FasL), triggers a signal resulting in apoptotic cell death. Fas is produced by breast epithelial cells, but its contribution to breast tissue homeostasis is unknown. This study investigated whether FasL is synthesized in the breast. By reverse transcription-polymerase chain reaction (RT-PCR), western blotting, and immunohistochemistry, FasL expression was analysed in normal and malignant human breast epithelial cell lines, normal breast tissue, benign breast disease (®brocystic changes, ®broadenoma), and breast cancer (ductal carcinoma in situ; invasive ductal, lobular, mucinous and medullary carcinomas). The results demonstrate expression of FasL by normal breast epithelial cells and show a marked increase of FasL protein in the majority of breast carcinomas, compared with normal breast tissue and benign breast disease. By western blot analysis, soluble FasL was detected in culture supernatants of one of three normal breast epithelial cell lines and in all four breast cancer cell lines tested. The expression of Fas protein was more heterogeneous in benign and malignant breast tissue, with expression levels ranging from weak to strong, but breast cancer cells frequently exhibited a weaker Fas expression than surrounding residual normal breast epithelial cells. In vitro, two out of three normal breast epithelial cell lines were sensitive to cell death induction by an agonistic anti-Fas antibody. Co-treatment with cycloheximide, an inhibitor of protein translation, rendered the resistant cell line sensitive. In contrast, two out of four breast cancer cell lines were resistant to the anti-Fas antibody and this resistance could not be reversed by cycloheximide. These results suggest that increased expression of FasL may confer an advantage on breast cancer cells, possibly by eliminating tumour-in®ltrating immune cells, and/or by facilitating tissue destruction during invasion.