The three mistletoe (Viscum album L.) lectins. ML I, ML II and ML III, were tested on their ability to enhance the secretion of the cytokines tumor necrosis factor (TNF)alpha, interleukin (IL)-1 alpha, IL-1 beta and IL-6 by human monocytes obtained from healthy donors. At lectin concentrations from 0.02 to 20/pg ml (100-10,000-fold lower than those showing toxic effects), stimulations of cytokine production several-fold over control values were observed. The immunoactivating concentrations by the three lectins were found different for each donor. At toxic concentrations, the amounts of IL-1 alpha, IL-1 beta and to a less extent of TNF alpha in monocytes supernatants were particularly high. The data are discussed in relationship with the cytotoxic and immunoactivating effects of mistletoe lectins and their interest in cancer treatment.