Galactoside-specific mistletoe lectin-1 (ML-1) was isolated by affinity chromatography from proprietary mistletoe extract and checked in BALB/c-mice for its immunoactive potency. To investigate the optimal immunomodulating dosage, ML-1 (0.5, 1.0, 2.5, 5.0 ng/kg body weight, b.w.) was subcutaneously administered for three subsequent days followed by another injection 48 h later. These studies proved that injections of 1 ng ML-1/kg b.w. induced optimal immunomodulation, since thymocyte proliferation, maturation and emigration were significantly enhanced in this murine model as compared to non-treated control mice. Further on, counts of peripheral blood lymphocytes and monocytes as well as expression of relevant activation markers on these cells revealed significant increases after ML-1 (1 ng/kg b.w.) administration. However, increase of cell counts and activity of peritoneal macrophages were less pronounced but still statistically significant for this ML-1 concentration. Determination of immune responses after low dose ML-1 treatment (0.5 ng/kg b.w.) presented relevant (partly statistically significant) increases, too. However, high dose ML-1 treatment (2.5, 5.0 ng/kg b.w.) did not enhance (but suppress) relevant immune functions. For future clinical/therapeutical treatment strategies, ML-1 dosages ranging from 0.5-1.0 ng/kg b.w. may be supposed to be optimal.