There is a growing interest in complementary medical practices, but few studies have investigated mechanisms behind the possible benefits. The effects of the homeopathic preparation Engystol on interferon-gamma producing T-lymphocytes were studied in vitro. Lymphocytes were isolated from 30 healthy human volunteers and the percentage of interferon-gamma producing cells was analysed by fluorescence activated cell sorting. Cells were treated with NaCl (control) or Engystol at concentrations from undiluted to 2%. All concentrations of Engystol increased the percentage of interferon-gamma producing lymphocytes significantly, from a mean of 20.9% +/- 10.5% to over 24%. There was no dose-dependence of the effect at the concentrations tested.