6′-Sialyllactose is a compound wherein the acetylneuraminyl (NANA) unit is connected to the galactosyl unit of lactose at the 6 position. In 3’-sialylactose, this connection is at the 3 position. 3′-Sialyllactose and 6′-Sialyllactose are used to differentiate and characterize binding domains of viruses, such as influenza and rhinitis viruses, that recognize NANA capped cell surface receptors. 3′-Sialyllactose and 6’-sialylactose are used as reference compounds in analytical methods developed to measure their presence in materials such as milk and colostrums.