abstract methods for the measurement of the minimum inhibitory concentration (mic) of antibacterial agents have been developed for water-soluble substances and require adaptation for use with water-insoluble essential oils. this paper reports a standardized agar dilution mic method, using 0.5% v/v tween 20 as a dispersant, which provides a reliable and reproducible technique. the method was tested using two melaleuca alternifolia oil samples with two different levels of terpinen-4-ol (37% and 45% v/v). the mic values of die tea tree oil samples against a wide selection of bacteria, moulds and yeast are reported.