Extracts from plants of Loranthaceae family collected from 15 host trees were screened for their cytocidal activity to tumour cells. Biologically active extracts prepared from Helicanthis elasticus grown on Mangifera indica and Citrus maxima (host trees) were partially purified by the gel filtration method. The purified material was found to be cytotoxic in vitro and reduced solid and ascites tumours in mice when given either simultaneously or prophylactically. The partially purified material was found to be a protein with a molecular weight of approximately 80,000 Da.