Thionins are small basic peptides found in different plant species, which are known to exert cytotoxic properties. In addition, previous data indicated an activation of human granulocytes by thionins from European mistletoe (viscotoxins, VT). To extend these latter findings, we investigated the influence of VT and from thionins from wheat flour (purothionin) on human granulocytes by flow cytometry and tried to characterise the involved molecular structures and mechanisms. Phagocytosis was determined by incorporation of FITC-labelled Escherichia coli and respiratory burst by oxidation of dihydrorhodamine 123 to rhodamine 123. VT and purothionin significantly enhanced E. coli-stimulated phagocytosis and respiratory burst at 25 and 250 microgram/ml. Phagocytosis of damaged lymphocytes by granulocytes was detected by electron microscopy in the VT-stimulated (100 microgram/ml) but not in the control cultures. The poly-cationic structure of the intact molecule seems to be crucial, as evidenced by comparison of the burst and phagocytosis-enhancing effects induced by other poly-cationic (protamine sulphate, histone, poly-l-arginine, poly-l-lysine) and poly-anionic (poly-l-glutamic acid) peptides, while pore forming due to amphipathic properties seems to be less important. Ca2+ and Mg2+ could not inhibit VT-enhanced phagocytosis and, thus, could not inhibit binding of VT to granulocytes. In addition, verapamil at low concentrations inhibited VT activity, suggesting the involvement of Ca2+ channels for granulocyte activation by the VT. Similarly, thionins and histones in contrast to protamine sulphate induced cell death of granulocytes at 250 microgram/ml as demonstrated by an enhanced release of reactive oxygen intermediates in unstimulated granulocytes. From these data one may suggest that activity of VT is induced by strong unspecific ionic binding, probably followed by specific receptor binding, and thionins exhibit stimulatory and cytotoxic effects on immune cells, which have to be further characterised.