A selection of essential oils and components were tested in vitro for potential immunomodulating effects on natural killer cell activity (NKCA) and lymphocyte activation through CD69 expression. Matricaria recutita , Boswellia carteri , Pelargonium graveolens , Lavandula angustifolia , Citrus limon , Melaleuca alternifolia , Melaleuca viridiflora , Santalum spicatum , Cedrus atlantica , and Thymus vulgaris ct. linalool essential oils were solubilised with ethanol and methylated β-cyclodextrin 1:5:25 (w/v) and pre-incubated at 10 μg/mL and 2 μg/mL ( M. alternifolia and C. atlantica : 2 and 1 μg/mL) with human peripheral blood mononuclear cells (PBMC) for 2 h. They were then incubated for 2 h with K562 target cells before determination of NKCA. The first 6 of these oils were similarly solubilised, pre-incubated for 2 h with aliquots of whole blood, then incubated with PHA for 4 h before measurement of CD69 antigen expression. Reference components α-pinene, S -(−)-limonene, linalool, geraniol, thymol, 1,8-cineole, linalyl acetate, eugenol, (+)-terpinen-4-ol, and trans -caryophyllene were solubilised with ethanol 1:5 and similarly tested. Dose-dependent stimulation of NKCA was found for trans -caryophyllene and linalyl acetate. No other effects of essential oils or components were found in these assays. Methylated β-cyclodextrin proved to be a less than ideal agent for enhancing solubility of essential oils, due to substantial effects on assay outcomes.