The effect of subcutaneous injection of physiological saline (given as 'placebo' in a randomized double-blinded placebo-controlled study) on immunocompetent cells from healthy individuals was analyzed. In two studies in 1998/1999 and 2002, 16 and 13 healthy individuals, respectively, were injected subcutaneously with 1 ml physiological saline twice a week for up to 12 weeks. Lymphocytes were isolated before and during exposure and incubated with recall antigens (purified protein derivative [PPD], tetanus toxoid [TT], bacillus Calmette-Guerin [BCG]). The production of T-helper type 1-, type 2-, and macrophage/monocyte-related cytokines was analyzed by ELISA. There was a significant increase of the recall-antigen-induced production of IFNgamma, IL-5, IL-13, TNFalpha, and GM-CSF in both groups during the observation period. Subcutaneous injection of placebo, therefore, enhances immunoreactivity. Psychological aspects, activation of the autonomous nerve system or local activation of mast cells or dendritic cells may be responsible for this phenomenon.