Using a plaque reduction assay, treatment of human influenza A viruses with the fruit-juice concentrate of Japanese plum (Prunus mume SIEB. et ZUCC) showed strong in vitro anti-influenza activity against human influenza A viruses before viral adsorption, but not after viral adsorption, with 50% inhibitory concentration (IC50) values against A/PR/8/34 (H1N1) virus, A/Aichi/2/68 (H3N2) virus and A/Memphis/1/71 (H3N2) virus of 6.35±0.17, 2.84±1.98 and 0.53±0.10 μg/ml, respectively. The plum-juice concentrate exhibited hemagglutination activity toward guinea pig erythrocytes. Its hemagglutination activity was inhibited by the monosaccharide N-acetylneuraminic acid and a sialoglycoprotein (fetuin), but not by the other tested monosaccharides (mannose, galactose, glucose and N-acetylglucosamine), suggesting the presence of a lectin-like molecule(s) in the Japanese plum-juice concentrate. Our findings suggest that the fruit-juice concentrate of Japanese plum may prevent and reduce infection with human influenza A virus, possibly via inhibition of viral hemagglutinin attachment to host cell surfaces by its lectin-like activity.