Drug resistance frequently occurs during the endocrine therapy of hormone-responsive breast cancer with antiestrogens. To investigate whether the antiestrogen-mediated induction of transforming growth factor-β (TGFβ) in mammary carcinoma could suppress the antitumor-immune response, autologous mixed lymphocyte tumor reactions (MLTR) with breast cancer biopsies were done. Antiestrogen treatment caused a TGFβ-dependent decrease of the immune cell–mediated antitumor-response, as seen by less apoptotic tumor cells (terminal deoxynucleotidyl transferase–mediated dUTP nick end labeling assay). For details, see the article by Joffroy and colleagues on page 1314 of this issue.