In milk, αs1-, αs2-, β-, and κ-caseins undergo association into colloidal complexes (casein micelles) that are visible with the electron microscope. Hydrophobic interactions and Ca2+ bonding are among the major causes of colloidal complex formation. In model systems, stable colloidal casein micelles can be obtained in a calcium caseinate solution by centrifugation at 1500 × g. The stabilities of colloidal complexes of bovine and two caprine caseins, selected for their αs1-casein contents, were tested and thermodynamically linked with the calcium-induced changes in the amount of stable colloid present. Analysis of the data according to this thermodynamic linkage approach for bovine and caprine caseins indicates colloid formation at low calcium concentrations (c.015 M). However, at increased calcium ion concentrations, these colloids are destabilized. Bovine casein (αs1-casein = 38% of total casein) was most stable with respect to added calcium ion concentration, and caprine casein, low in αs1-casein (5% of total casein), was least stable. The high caprine (αs1-casein = 17% of total casein) was intermediate in stability. After destabilization, bovine casein was not resolubilized at elevated calcium concentrations, but both caprine caseins were. More casein from the low αs1-casein sample could be resolubilized (salted in). These results suggest a role for casein composition in dictating the functional properties of milks from various species. © 1993, American Dairy Science Association. All rights reserved.