The immunomodulatory effects of Iscador (a mistletoe, Viscum album extract) were investigated. After a single intravenous infusion of Iscador several immunological parameters in the peripheral blood of breast cancer patients were examined. Parallel with neutrophilia, and with an increase of juvenile neutrophils, a significant enhancement of phagocytic activity of granulocytes was shown. After short decreases during the first 24 h, significant increases in natural killer (NK) and antibody-dependent cell-mediated cytotoxicity (ADCC) activities as well as augmented levels of large granular lymphocytes were observed. These NK/ADCC responses followed a kinetic pattern similar to that after treatment with α-interferon described by others. Further significant increases in mitogenic responses to phytohemagglutinin and concanavalin A were observed.