Direct and indirect calorimetry were used to determine the metabolic rates of the bacterium Escherichia coli and the baker's yeast Saccharomyces cerevisiae under the influence of five tea-tree oils and some of their constituents. Bacteriostatic and bactericidal as well as fungicidal effects were observed, dose-effect curves established, and the kinetics of the inactivation evaluated. Experiments showed that subpopulations of the E. coli suspensions were less sensitive against these essential oils than the whole population and were able to continue to metabolize and to grow. Some general information about tea-tree oils, their origin, application and composition is given.