Controlled activation of non-specific and specific immune defence mechanisms can beneficially manipulate the host's ability to attack malignant cells. In this context, migration and tissue distribution of immunocompetent cells may be prerequisites for an efficient immune surveillance. The effect of various non-cytotoxic concentrations of the Viscum album L. (mistletoe) preparation Iscadore QuFrF on the locomotory activity of immunomagnetically isolated human CD4+ and CD8+ T lymphocytes from healthy donors was investigated. Cellular migration was examined within a three-dimensional collagen matrix. Donor-dependent variations in baseline activities of spontaneously locomoting T cells were accompanied by individual response patterns of T cells from different donors in the presence of various concentrations of mistletoe preparation (0.25-2.5 micrograms/ml). Using the three-dimensional collagen matrix assay an induction of locomotory activity was detected in a highly reproducible fashion although the optimal concentration of mistletoe preparation and the time point of maximal response were individual for each donor. Our data suggest that the direct stimulation of T-cell migration by mistletoe components may modulate the system of immune surveillance and recognition in patients under mistletoe therapy.