Macadamia integriflora (family Proteaceae) is an endemic Australian plant that has been used for thousands of years as a food. Its nuts are known to keep well, raising the possibility that they may contain antimicrobial compounds and therefore may have value as a functional food to retard food spoilage and prevent food poisoning and other food-borne diseases. M. integriflora extracts were investigated for their ability to inhibit the growth of a panel of bacteria and fungi of importance to food spoilage and food poisoning, as well as the gastro-intestinal protozoal parasite Giardia duodenalis. All extracts displayed broad spectrum antimicrobial activity, each inhibiting the growth of 7 of the 18 bacterial species tested (39%). In contrast, none of the fungal species were inhibited by the macadamia nut extracts. Strong inhibitory activity was detected with MIC values as low as 0.80 µg/ml against some bacteria, although most measured MIC's were generally several orders of magnitude higher than this. All extracts were more effective against Gram-negative than Gram-positive bacteria. Indeed, all extracts inhibited 54 % of the Gram-negative bacteria tested and none of the Gram-positive bacteria. All extracts were also effective in inhibiting the gastro-intestinal protozoan parasite G. duodenalis, yet were nontoxic in the Artemia franciscana bioassay with LC50 values greatly in excess of 1 mg/ml. The inhibitory bioactivity against a range of microbes, as well as the lack of toxicity, indicates the potential of macadamia nuts in the discovery and development of new natural food preservatives and pharmaceuticals.