Glycomacropeptide (GMP) was purified from sweet whey dialyzed in water by anion-exchange chromatography on DEAE-Sephacel at pH 2.0-4.5. The optimum pH range was 2.5-4.0. The yield of purified GMP increased and its sialic acid concentration decreased with increasing pH value. The GMP had an apparent isoelectric point < 3.8. Dialysis of sweet whey was shown to be important to maximize the yield of GMP adsorbed to the anion-exchanger. Only highly sialylated GMP, accounting for approximately 55% of total sialic acid content, was adsorbed on the anion-exchanger from non-dialyzed sweet whey.