Purothionin from wheat flour was chemically modified by acetic or succinic anhydride under specific conditions. The complete modification of all amino groups of purothionin caused a large change in the net charge of the molecule, leading to the loss of the toxicity to mice and yeast. The sole tyrosyl residue in purothionin was nitrated by tetranitromethane at neutral pH or iodinated by the lactoperoxidase method. The nitro- and diiodo-derivatives of purothionin showed considerably reduced toxicity. Based on these modification studies we conclude that the positive charges of lysyl residues have an important role in the interaction with the negatively charged cell surface, and that the emergence of the toxicity of purothionin depends on a certain state of the tyrosyl residue.