A new family of antimicrobial proteins is described. Prototype proteins can be isolated from Macadamia integrifolia as well as other plant species. DNA encoding the protein is also described as well as DNA constructs which can be used to express the antimicrobial protein or to introduce the antimicrobial protein into a plant. Compositions comprising the antimicrobial protein or the antimicrobial protein per se can be administered to plants or mammalian animals to combat microbial infestation.