Lactoferrin is an 80-kDa, iron-binding glycoprotein present in milk and, to a lesser extent, in exocrine fluids such as bile and tears. It consists of a single-chain polypeptide with two gobular lobes and is relatively resistant to proteolysis. The complete cDNAs for lactoferrin from human milk, neutrophils, and bovine milk have been reported, and recombinant proteins have been produced. Owing to its iron-binding properties, lactoferrin has been proposed to play a role in iron uptake by the intestinal mucosa and to act as a bacteriostatic agent by withholding iron from iron-requiring bacteria. Its presence in neutrophils and its release during inflammation suggest that lactoferrin is also involved in phagocytic killing and immune responses. Additionally, lactoferrin may function in ways not related to iron-binding, e.g. as a growth factor and as a bactericidal agent. This review attempts to evaluate these proposed functions and their biological significance in more detail.