The subcutaneous application of lectin-rich mistletoe preparations such as Iscador Quercus (IQ; Weleda Company, Schwäbisch Gmünd, Germany) results in a peripheral eosinophilia. Our goal was to investigate whether this effect is related to mistletoe lectin (ML) and whether it is caused by a response of the specific immune system. In a double-blinded study, 43 volunteers were randomized to one of four treatment groups: (1) IQ, (2) ML that was derived from IQ, (3) IQ that was depleted of ML, and (4) placebo. The respective preparations were applied subcutaneously twice per week for 8 weeks, in increasing doses. Weekly the differential blood count was analyzed. Every 4 weeks interferon-gamma, interleukin-5 (IL-5), and granulocyte-macrophage colony stimulating factor (GM-CSF) were determined in cultures from peripheral mononuclear cells after stimulation with IQ. IQ and ML resulted in significant eosinophilia compared with placebo and ML-depleted IQ. Furthermore, the leukocyte and granulocyte counts were increased in the IQ and ML groups compared with placebo. GM-CSF, interferon-gamma, and IL-5 increased after ex vivo in vitro stimulation with IQ in the IQ and ML groups, and were significantly different from placebo in the IQ group but not in the ML group. Eosinophilia during therapy with mistletoe preparations is due to its content of ML. This effect might be related to a stimulation of IL-5 and/or GM-CSF, which was demonstrated ex vivo in vitro. ML resulted in a temporary increase of the granulocyte count, which is probably related to an acute-phase reaction.