Thionins are low-molecular-weight proteins (M(r) ca. 5000) occurring in seeds, stems, roots and leaves of a number of plant species. The different members of this family of plant proteins show both sequence and structural homology, and are toxic to bacteria, fungi, yeasts and various naked cells in vitro. Toxicity requires an electrostatic interaction of the positively charged thionin with the negatively charged phospholipids making up the membrane, followed by either pore formation or a specific interaction with a certain lipid domain. This domain might be composed of phosphoinositides, which mediate transduction of environmental signals in eukaryotes. Their in vitro toxicity to plant pathogenic bacteria and fungi could reflect a direct role in plant defence, although, in view of the many divergent activities displayed by thionins both in vitro and in vivo, a biological role other than inhibition of microbial growth is equally plausible.