The interaction of lectin I from mistletoe (Viscum album) (ML I) and its isolated A and B chains with mononuclear cells from healthy human donors was investigated with respect to proliferation capacity (mitogenicity) and monokine/lymphokine factor (MSF) production. The factor produced was studied by means of the electrophoretic mobility inhibition assay using guinea pig macrophages as indicator cells. ML I and its B chain exhibited comparable inhibitory effects on the proliferation of mononuclear cells (MNC, lymphocytes) that differed in quantity only. After 72 h the tritiated thymidine incorporation had diminished in comparison to the control over a concentration range from 10(-7) to 10(-11) mol/l (ML I) or from 3 X 10(-7) to 3 X 10(-9) mol/l (B chain), respectively. The ML I was about 30 times more active than the B chain. Moreover, MSF production could be substantiated for the B chain at a concentration of 3 X 10(-8) mol/l. In contrast to the B chain, the A chain stimulated the MNC to blastogenic transformation at concentrations of 3 X 10(-8) and 3 X 10(-9) mol/l. The stimulation index was much lower than after PHA stimulation. The same concentrations induced the production of lymphokine (MSF). The lymphokine activity on the indicator cells could be inhibited by L-fucose. Perhaps both cytotoxicity and lymphocyte stimulation are important for anti-tumor activity after application in vivo.