The ability of immunomodulating mistletoe extract standardized for the galactoside-specific lectin (ML-1) to affect immunological parameters (peripheral blood lymphocyte counts, cytokine release) as well as neuroendocrinological parameters (beta-endorphin release) was investigated in breast cancer patients (n = 36). Regular subcutaneous injections of the optimal immunomodulating ML-1 dosage (1ng/kg body weight, twice a week) for 12 weeks induced 1) a significant increase (p < 0.005) of beta-endorphin plasma levels, 2) a reduced decrease (respectively moderate increase) of defined peripheral blood lymphatic subsets after standard chemotherapy, 3) an evidently increased in vitro cytokine release by mononuclear immune cells after adequate stimulation. The increased levels of plasma beta-endorphin after ML-1 administration obviously correlate with an improved quality of life in this group of patients; however, increased in vitro cytokine release and stabilization of peripheral blood lymphocyte counts after chemotherapy demonstrate the immunoactive potency of ML-1.