Subtypes of transferrin C were studied by means of isoelectric focusing after complete desialylation of transferrin. Family data were consistent with an autosomal co-dominant mode of inheritance. Studies of serum samples from 75 individuals heterozygous for C and another (B or D) variant showed that the genes (C1 and C2) controlling the C subtypes are allelic to the B and D genes. The C2 gene frequency in Swedes and Swedish Lapps was similar to that found previously in Danes and Germans.