The antibacterial activity of 14 essential oils and their major constituents in the gaseous state was evaluated against Haemophilus influenzae, Streptococcus pneumoniae, Streptococcus pyogenes and Staphylococcus aureus. For most essential oils examined, H. influenzae was most susceptible, followed by S. pneumoniae and S. pyogenes, and then S. aureus. Penicillin-susceptible and -resistant S. pneumoniae were comparable in susceptibility. Escherichia coli, which was used as a control, showed least susceptibility. A minimal inhibitory dose (MID) was introduced as a measure of the vapour activity. Among 14 essential oils, cinnamon bark, lemon-grass and thyme oils showed the lowest MID, followed by essential oils containing terpene alcohols as major constituents. The essential oils containing terpene ketone, ether and, in particular, hydrocarbon had high MIDS. The vapour activity on short exposure was comparable to that following overnight exposure, and rapid evaporation was more effective than slow evaporation of essential oils. The vapour concentration and absorption into agar of essential oils reached a maximum 1 or 2 h after rapid evaporation. These results indicate that the antibacterial action of essential oils was most effective when at high vapour concentration for a short time.