SummaryIn this paper family studies are presented which support the hypothesis of polymorphism in the process controlling sialic acid binding to bovine transferrin which modifies its phenotype as seen in starch gel electrophoresis. It has been shown that this polymorphism is controlled by a locus Tfs with two allelesTfs AandTfs a. Tfs a/aanimals have the abnormal phenotype with the two faster bands of the four bands of a normal transferrin allele being virtually absent.Tfs A/aandTfs A/Aare phenotypically normal. Limited evidence is presented which suggests that the Tf and Tfs loci are not linked.