Analytical isoelectric focusing of human serum transferrin has revealed the presence of up to nine different transferrin subfractions. The less prevalent subfractions have hitherto not been available for analytical work on their composition. Prolonged isoelectric focusing in immobilized pH gradients is capable of effectively separating and concentrating these subfractions from other fractions that are often present in over 70-fold excess in native preparations. Preliminary results of the carbohydrate analysis indicate a heterogeneity extending beyond the present concepts of the transferrin glycan structure.