The effect of different essential oils as well as of isolated mono- and sesquiterpenes on the viability of bloodstream forms of Trypanosoma brucei, promastigotes of Leishmania major and human HL-60 cells was evaluated using the Almar Blue assay. Of the 12 essential oils and 8 terpenes investigated, only three essential oils, Melissa officinalis (balmmint) oil, Thymus vulgaris (thyme) oil, and Melaleuca alternifolia (tea tree) oil were about 50-fold and 80-fold more toxic to bloodstream forms of T. brucei than to HL-60 cells, respectively. Terpinen-4-ol, the main compound of the Australian tea tree oil, was even 1000-fold more toxic to trypanosomes than to the human cells. On the other hand, none of the essential oils and terpenes tested were more toxic to promastigotes of L. major than to HL-60 cells.