The human serum transferrin (Tf) system was analyzed by isoelectric focusing (IEF) with immobilized pH gradients. For the demonstration of the genetic variability the Fe1-Tf region was chosen. The pH range suitable for analysis of the Tf system was pH 5.20 to pH 5.75. The phenotypes of the common six TfC subtypes are described. No further heterogeneity among TfC1, TfC2, and TfC3 was noted. Aslo presented are the phenotypes of the TfC6 subtype, and of three different TfB and three different TfD variants. IEF with immobilized pH gradients appears to be a suitable method for the analysis of the inherited transferrin polymorphism.