Abstract: Novel cytotoxic agent useful against malignant tumors. It provides a stable composition of matter based on the cytotoxic activity of two synergistically acting toxins which sequence are herein described. The basic amphipathic peptide binds to the cell membrane causing perturbation of the lipid bilayer. The non-covalent heterodimer complex dissociates and the phospholiphase A.sub.2 subunit (B) binds to the cell membrane. Subunit A acts as a chaperon preventing non-specific binding of the phospholipase A.sub.2, it has no enzymatic activity. The basic amphipathic peptide increases the effect of the phospholipase A.sub.2 subunit. Cell death is caused by the enzymatic hydrolysis of cell membrance phospholipids. Both the non-covalent heterodimer complex and the basic amphipathic peptide used were purified from the venoms of Crotalus durissus terrificus and Naja naja atra, respectively.