Abstract: The present invention provides a stable composition of matter based on the cytotoxic activity of a basic phospholipase A.sub.2 of molecular weight 14,500 and isoelectric point 9.6-9.7 (crotoxin B) isolated from the venom of Crotalus durissus terrificus which in complex with a specific, non-enzymatic, peptide of molecular weight 9,500 and isoelectric point 3.5-3.7 (crotoxin A) displays a preferential cytotoxic activity against various types of tumor cells. When administered parenterally in an acceptable vehicle and in pharmacologically efficient amounts to animals and humans the complex is useful in the treatment of malignant tumors in advanced stages. The method for purification of the active components, the preparation in a pharmacologically acceptable form, and the method of therapeutic use of the present composition of matter are also disclosed.