Milk is a heterogeneous fluid in which the colloidal phase is homogeneously dispersed in the aqueous phase. Calcium is partitioned between the colloidal and aqueous phases and is in complex electrochemical equilibrium with several major milk components. In human and bovine milk, calcium is mainly distributed between the aqueous and casein micelle in the colloidal phases (Holt & Jenness, 1984; Neville et al. 1994). Caseins form a complex micelle structure that contains approximately 25000 phosphorylated monomers that react with calcium phosphate complexes in the milk to bind 20–40 mole calcium per mole casein (Holt & Jenness, 1984; Neville et al. 1994). Thus, the distribution of calcium between the colloidal and aqueous phases appears to be governed by the level of casein in the milk (Holt & Jenness, 1984; Neville et al. 1994). In human milk, the casein level is low; ∼25% of calcium is associated with casein, whereas in cows and goats the corresponding figure is higher at ∼65%. In rats, casein levels are among the highest in mammals and 95% of calcium is associated with casein (Neville et al. 1994). In the aqueous phase, calcium is divided among ionic calcium (Ca2+), calcium citrate and calcium phosphate. Calcium citrate and calcium phosphate constitute most of the aqueous calcium in bovine milk in contrast to less than 50% in human milk (Holt & Jenness, 1984; Neville et al. 1994). As the concentration of Ca2+ in milk appears to be essential in preserving the integrity of the mammary tight junctions during lactation (Neville & Peaker, 1981), it is important to follow its concentration precisely in mammary secretions in different stages of the reproduction cycle.