Phytoncides increase natural killer (NK) cell activation and cytotoxicity in NK-92mi cells. Human NK-92mi cells were activated by incubation with phorbol 12-myristate 13-acetate (PMA) and ionomycin for 6 h after treatment with α-pinene, terpinolene, o-cymene and camphor at 100 μM. The expression levels of (a) perforin, (b) CD56, (c) granzyme B and (d) CD107a mRNA were analyzed with real-time PCR. The data are expressed as the mean ± SD of three independent experiments (* p < 0.05, *** p < 0.005). Human NK-92mi cells were incubated with target K562 cells or HEC-1A cells, which were prelabeled with calcein-AM for 4 h, as described. Calcein-AM released from K562 and HEC-1A target cells was measured by a fluorescence microplate reader. (e) NK-92mi cytotoxicity towards Hec-1A cells. (f) NK-92mi cytotoxicity towards K562 cells. Con: control, Con Pi: control with α-pinene treatment, P+I: PMA and ionomycin activation, P+I Pi: PMA and ionomycin activation with α-pinene treatment. Results are expressed as the mean ± SD of three independent experiments (* p < 0.05, ** p < 0.01, *** p < 0.005).