Small acidic protein a-lactalbumin, one of the major protein components of milk, is one of the most extensively investigated Ca2+-binding proteins, which does not belong to the EF-hand family of calcium-binding proteins. It serves as a model for studies of the mechanisms of protein stability, folding and unfolding. a-Lactalbumin acts as a regulatory subunit of galactosyltransferase in lactose synthase, which catalyses the synthesis of lactose from UDP-galactose and glucose. It represents a classical example of molten globule state at acidic pH and in its apo-form at elevated temperatures. Three-dimensional structures of several ?-lactalbumins are determined. The protein possesses a single strong Ca2+-binding site, which binds Mg2+, Mn2+, Na+, and K+ as well, and several distinct Zn2+-binding sites. The binding of cations to the Ca2+-site increases protein stability against action of heat, various denaturing agents and proteases, while the binding of Zn2+ to the Ca2+-saturated protein decreases its stability and causes its aggregation. ?-Lactalbumin interacts with membranes, proteins, peptides and low molecular weight substrates and products. These interactions are modulated by the binding of metal cations to a-lactalbumin. a-Lactalbumin forms amyloid fibrils at low pH values and some folding variants of a-lactalbumin demonstrate bactericidal activity and some of them cause apoptosis of tumour cells. Thus, ?-lactalbumin is a metal binding protein, the function of which depends on its environment: it takes part in lactose synthesis in the mammary gland and could be important for lowering the incidence of cancer and various infections in breast-fed children.