Using a combination of gel filtration and paper chromatography, a tumour reducing component from mistletoe extract (Iscador) was isolated and identified to be a peptide of approximate molecular weight 5000. The isolated peptide reduced the solid tumour induced by Dalton's lymphoma ascites tumour cells (DLA cells) in mice. The isolated component was very cytotoxic to the DLA cells but was not cytotoxic to normal lymphocytes, indicating a cell dependent specificity.