The invention provides modified forms of bouganin protein having biological activity and a reduced propensity to activate human T cells as compared to the non-modified bouganin protein. The invention also provides T-cell epitope peptides of bouganin, and modified T-cell epitope peptides of bouganin which have a reduced propensity to activate human T cells as compared to the non-modified T-cell epitope peptide. The invention also provides cytotoxins having the having a ligand that binds to a cancer cells attached to the modified bouganin proteins. Also provided are methods of inhibiting or destroying mammalian cancer cells using the cytotoxins of the invention and pharmaceutical compositions for treating human cancer.