Some Egyptian plants were screened against highly pathogenic avian influenza strain H5N1 using plaque inhibition assay in Madin-Darby canine kidney. The results indicated that the extracts of Red Sea grass Thallasodendron ciliatum possessed potent antiviral activity (100% inhibition at the concentration of 1 μg mL⁻¹). The bioactivity-guided fractionations led to the isolation of a new diglyceride ester (1) along with asebotin (2) for the first time from the plant. The two isolates showed reduction of virus titre by 67.26% and 53.81% inhibition at concentration of 1 ng mL⁻¹, respectively.