Garcinia mangostana (family Clusiaceae), commonly known as mangosteen, is a large tree native to tropical regions of Asia. Its fruit are highly prized for their pleasant taste. The fruit pericarp also has ethnobotanical uses in the treatment of a wide variety of diseases and medical conditions. G. mangostana fruit pericarp extracts were investigated for their ability to inhibit the growth of a panel of bacteria and fungi of medicinal importance. The extracts were also tested for their ability to block the proliferation of the Jeg-3 human choriocarcinoma cell line. All extracts displayed broad spectrum antimicrobial activity, inhibiting the growth of between 13 (72%) and 15 (83%) of the 18 bacterial species tested. In contrast, none of the fungal species were inhibited by the mangosteen extracts. Strong inhibitory activity was detected with minimum inhibitory concentration (MIC) values as low as 10 μg mL-1 against some bacteria. All extracts were effective against both Gram negative and Gram positive bacteria, although Gram negative bacteria were more sensitive. All extracts displayed anti-proliferative activity against Jeg-3 human choriocarcinoma cells, yet were non-toxic in the Artemia franciscana bioassay with LC50 values greatly in excess of 1 mg mL-1. The inhibitory bioactivity against a range of microbes, the antiproliferative activity against Jeg-3 cells, as well as the lack of toxicity, indicate the potential for mangosteen in the discovery and development of new natural pharmaceuticals.