The in vitro antiproliferative or stimulatory activity of an aqueous mistletoe extract (AME) with a defined content of bioactive mistletoe lectin (ML) was investigated in 6 human tumor cell lines, including two melanomas and leiomyosarcomas, each of which had previously been reported to show evidence of growth stimulation if treated with low concentrations of isolated ML. The effects of AME were compared to that of the standard cytotoxic agent adriamycin (ADR) using the well established propidium iodide and sulforhodamin B proliferation assays. The AME concentrations used ranged from 0.5 pg to 5 ng (0.82 fMol-85 pM) bioactive ML/ml in melanoma (HT-144, SK-MEL-28) and leiomyosarcoma (SK-MLS-1, S-UT-1B) cell lines and from 0.1-100 ng ML/ml (1.7 pM-1.7 nM) in MCF-7 breast cancer and SW620 colon carcinoma cell lines, respectively. The influence of AME on cell growth was determined at various time-points from 24 hours to 6 days of exposure. We found a time- and cell line-dependent inhibition of tumor cell growth, but no reproducible stimulation of tumor cell proliferation. Inhibitory concentrations 50% (IC50) for e.g. the SK-MEL-28 melanoma cell line, decreased from 4.1 ng ML/ml at 24 hours to 0.16 ng ML/ml at 72 hours and 0.18 ng ML/ml at 5 days. Our data clearly demonstrate that, by applying scientifically valid methods and procedures, the standardized AME did not stimulate tumor cell proliferation but showed time- and concentration-dependent antiproliferative effects.