Disclosed herein is a process for the efficient production of .kappa.-casein GMP on an industrial scale. The production of high-purity .kappa.-casein GMP is realized by using an effluent of rennet-casein curd free of whey proteins, lactose and the like as a starting effluent, adjusting the pH of the effluent to an acidic range to remove calcium phosphate and then subjecting the resulting effluent to a desalting treatment. .kappa.-Casein GMP is used as a standard substance for the assay of sialic acid. It has also attracted attention for its physiological activities and other properties.