Lactoferrin (Lf-α) is a major constituent of the secondary granules of neutrophils, and is thought to be involved in bacteriustasis through high affinity binding of plasma Fe required for microbial growth. Lf-α is also the major Fe binding protein in milk. Fe bmmd to Lf-α is available for metabolic use, but the mechanism of Fe release from Lf-α is unlmown. Treatment of hmrmn Lf-α with neuraminidase to remove sialic acid residues reduced its ability to bind Fe and caused release of Fe already hotrod to the protein. Reduction in Fe binding capacity was dependent on time of exposure to and concentration of neuramim'dase, and was not due to Lf-α loss or degradation. Readdition of sialic acid to desialylated Lf-α using α - 2, 6 - sialyliransferuse restored Fe binding. The results suggest that sialylationdesialylation reactions could play a role in physiologic binding and release of Fe from the very high affinity binding sites of Lf-α.