Viscum album L. (VAL) is a phytopreparation used in adjuvant cancer therapy with both immunostimulatory and DNA stabilizing properties at low drug concentrations and cytostatic/cytotoxic properties at higher concentrations. The present work examines the cytotoxic effects of VAL extracts produced from mistletoes grown on different host trees and of purified toxic proteins from VAL, such as the D-galactose-specific lectin I (ML I), the N-acetyl-D-galactosamine-specific ML II and ML III, and crude viscotoxins towards cultured human lymphocytes. The decrease in the number of cultured lymphocytes and blast cells treated with whole plant extracts from VAL was host tree-specific. Nevertheless, there was no close correlation to the content of MLs or viscotoxins. Using the purified proteins, it became obvious that the cell killing was mediated by the induction of apoptosis, as measured by the appearance of a hypodiploid DNA peak using flow cytometry. ML III was the most effective to induce apoptosis, followed by ML II and ML I, while the viscotoxins and oligosaccharides from VAL did not. By measuring the surface expression of IL-2R alpha chains, transferrin receptors and APO-1/Fas molecules on non-apoptotic T cells, no significant changes were observed at low ML concentrations (1 ng/ml), but their decrease at higher ones. Our findings suggest that there might be at least two different ways of cell killing operative in VAL-mediated cytotoxicity: (a) the typical apoptotic cell death with the appearance of hypo-diploid nuclei, and (b) a direct or indirect killing by damaging the cell membrane with subsequent influx of Ca2+ and of the DNA intercalating dye propidium iodide and cell shrinkage. These effects might not be exclusive, as they probably occur simultaneously.