A placebo-controlled study of the effect of an Eleutherococcus senticosus extract (Eleukokk) on the immune system was performed with 36 healthy volunteers utilising quantitative multi-parameter flow cytometry with monoclonal antibodies directed against specific surface markers of human lymphocyte subsets. Volunteers in the verum group received 10 ml of an ethanolic (vincamine free) eleutherococcus senticosus preparation, 3 times daily for 4 weeks. In the placebo, the eleutherococcus extract was substituted by additional wine, resulting in identical final concentrations of ethanol in both preparations. The purpose of the double-blind study was the demonstration of possible effects on the cellular immune status, as determined by quantitative flow cytometry. The most salient feature in the verum group was a drastic increase in the absolute number of immunocompetent cells, with an especially pronounced effect on T lymphocytes, predominantly of the helper/inducer type, but also on cytotoxic and natural killer cells. In addition, a general enhancement of the activation state of T lymphocytes was observed. No side effects were observed during the trial or afterwards (observation period 6 months).