Antiestrogens (AE) like tamoxifen belong to the most commonly used drugs in the therapy of estrogen receptor positive breast cancer. Nevertheless, relapses frequently occur during treatment indicating development of drug resistance. Concerning the underlying mechanisms, the finding of AE induced TGFss in breast cancer cells appears to be relevant both with respect to TGFss's cell autonomous function as a mediator of cancer progression and its function as an immune regulator. To investigate the potential role of tumor derived, AE induced TGFss as a suppressor of the host's immune response, heterologous and autologous in vitro assays have been established in this work. Application of AE led to an impaired antitumor immune response, which could be restored by a neutralizing TGFss antibody. The work at hand thus provides evidence for an AE treatment induced, TGFss driven immunosuppression in the tumor microenvironment, a mechanism that could critically contribute to development of AE resistance during the