In Europe, commercially available extracts from the white-berry mistletoe (Viscum album L.) are widely used as a complementary cancer therapy. Mistletoe lectins have been identified as main active components and exhibit cytotoxic effects as well as immunomodulatory activity. Since it is still not elucidated in detail how mistle toe extracts such as ISCADOR communicate their effects, we analyzed the mechanisms that might be responsible for their antitumoral function on a molecular and functional level. ISCADOR-treated glioblastoma (GBM) cells down-regulate central genes involved in glioblastoma progression and malignancy such as the cytokine TGF-β and matrix-metalloproteinases. Using in vitro glioblastoma/immune cell co-cultivation assays as well as measurement of cell migration and invasion, we could demonstrate that in glioblastoma cells, lectin-rich ISCADOR M and ISCADOR Q significantly enforce NK-cell-mediated GBM cell lysis. Beside its immune stimulatory effect, ISCADOR reduces the migratory and invasive potential of glioblastoma cells. In a syngeneic as well as in a xenograft glioblastoma mouse model, both pretreatment of tumor cells and intratumoral therapy of subcutaneously growing glioblastoma cells with ISCADOR Q showed delayed tumor growth. In conclusion, ISCADOR Q, showing multiple positive effects in the treatment of glioblastoma, may be a candidate for concomitant treatment of this cancer.