The inhibitory activity of four essential oils against three fungi, two bacteria and a mammalian cell line was determined by gaseous and solution contact. Among the four oils, cinnamon bark oil showed the highest antimicrobial activity followed by thyme, perilla and lavender oils by both gaseous and solution contact. However, the growth of test organisms was inhibited more efficiently by gaseous contact than by solution contact, indicating the significant contribution of the direct vapour effect on the antimicrobial activity. The rank of the cytotoxicity against Chinese hamster cells was not in agreement with that of the antimicrobial activity. Thyme oil showed the highest toxicity by gaseous contact, but the least toxicity by solution contact. Lavender oil showed relatively high toxicity by solution contact compared with gaseous contact. Based on the enhanced bioactivity by gaseous contact, cinnamon bark, perilla and lavender oils may be used favourably in the gaseous state, while thyme oil may be used in aqueous medium due to its reduced toxicity in solution and enhanced toxicity in the vapour state.