The essential oil of leaves of Fortunella margarita is rich in the sesquiterpenoids β-eudesmol (28.3%), α-muurolene (10.3%), β-gurjunene (10.0%), γ-eudesmol (8.4%), and γ-muurolene (6.6%) while the essential oil extracted from the fruits showed monterpenoids as the main components, α-terpineol (55.5%), carvone (5.7%), and carveol (5.5%). Both samples were tested for antiviral activity against avian influenza (H5N1) virus, and the obtained results revealed that the fruit essential oil was more effective (80% virus inhibition by the MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay using Madin−Darby canine kidney (MDCK) cells for virus propagation). The IC50 values obtained for the leaf and fruit essential oils were 38.89 and 6.77 μg/mL, respectively [53].