The effects of mistletoe lectin I (ML I) on the human T-cell leukemia line MOLT-4, the monocytic line THP-1 and on human peripheral blood mononuclear cells (PBMC) were investigated with regard to general cell viability and induction of apoptosis. Using a sensitive serum-free cytotoxicity assay, the time- and concentration-dependent direct toxicity towards MOLT-4 cells was determined with IC50-values ranging from 20-40 pg/ml (300-600 fmol/l). Investigations on the time course of the toxic effect using selected concentrations of ML I revealed distinct response curves for concentrations of high, low and intermediate toxicity, respectively. The ratio of apoptotic to viable MOLT-4 cells was determined after treatment with ML I for 24 h. Apoptosis and cytotoxicity were correlated at low and intermediate concentrations, whereas at long intervals and high concentrations of ML I mostly necrotic effects were observed. The data showed that in the concentration range of low cytotoxicity ML I-induced cell death is quantitatively due to apoptotic processes. The immunomodulatory activity of ML I was investigated in vitro by measuring cytokine release. At concentrations of low cytotoxicity ML I showed immunostimulatory activity on PBMC and THP-1. RT-PCR with THP-1 cells confirmed that cytokine induction by ML I is regulated on the transcriptional level. These findings suggest that in the blood cells investigated both apoptosis and cellular signalling are induced by the same concentration range of ML I.