Migration and tissue distribution of immunocompetent cells may be critical prerequisites for efficient immune surveillance. The effect of various concentrations of the mistletoe extract Iscador QuFrF on the locomotory behavior and viability of immunomagnetically isolated human CD4+ and CD8+ T lymphocytes within three-dimensional collagen gels was investigated. Although variation in baseline activities of spontaneously migrating T cells was donor-dependent, a dose-dependent stimulation of the locomotory activity in both CD4+ and CD8+ T cells for noncytotoxic concentrations of Iscador QuFrF (0.25-1.25 micrograms/ml) was detected. The optimal concentration of mistletoe extract and time of maximal response were specific for each donor. As shown by cell tracking and subsequent data analysis, CD4+ T cells exposed to the mistletoe extract displayed a significant increase in mean velocity and time locomoting; total distance migrated was nearly doubled. In contrast, CD8+ T cells showed less pronounced changes in these critical parameters. Cytotoxic effects of the mistletoe preparation on T lymphocytes, which could at least partially be attributed to the induction of apoptosis, were drastically reduced in the presence of fetal calf serum in the culture system. Our data suggest that the direct stimulation of T-cell migration in the presence of mistletoe components may modulate in a dose-dependent manner the system of immune surveillance and recognition in patients under mistletoe therapy.