In course of a clinical breast cancer study, the influence of the chemotherapy on the immune system was investigated. The toxicity of the chemotherapeutics Epirubicin and Docetaxel, which were used in the study, were compared during this thesis. Epirubicin was shown to affect the metabolism (EZ4U-cytotoxicity-assay) and the function (IFNγ- ELISA) of PBMCs (Peripheral Blood Mononuclear Cells) in vitro to a greater extent than Docetaxel. The in vivo data showed no difference between the Epriubicin+Cyclophosphamid(conventional) and Docetaxel(revers) treated patients regarding the percentage of IFNγ producing CD8+ cells and the white blood cell counts. The analysis of the leukocyte subpopulations, showed a significant decrease in the cell counts of B-lymphocytes T-lymphocytes and natural killer cells of Epirubicin+Cyclophosphamide treated patients, according to the in vitro data.