Viscum album (VA) preparations (Iscador) consist of aqueous extracts from different types of European mistletoe. Biologically active components of VA extracts include mistletoe lectins (ML) and viscotoxins. The treatment with VA extracts or with purified ML has been shown to be associated with tumor regression in several in vivo experimental models of tumoral implantation. The mechanisms underlying the anti-tumoral activity of VA or ML are complex and involve apoptosis, angiogenesis and immunomodulation. This review provides an account of the current status of the understanding of the VA-associated immunomodulation in various cell types including lymphoblastoid, monocytic or endothelial cell lines.