The formation of protein polymers in ultra-high-temperature (UHT)-treated milk during storage at various temperatures was examined by gel filtration. The extent of polymer formation was found to depend on both storage time and temperature. After some months of storage at the higher temperatures of 30 and 37°C, the extent of polymerization of the caseins and whey proteins due to reactions of the Maillard type was several times greater than the heat-induced changes resulting from the UHT processing itself. After storage for 6 months the following proportions of milk proteins were found to exist in the form of covalently bound polymers: 50% at 37°C, 40% at 30°C, 26% at 20°C and 21% at 4°C. In addition, further amounts of polymer were formed by disulphide bonding, the contribution of such polymers diminishing gradually in a temperature-dependent manner during storage due to continuing polymerization reactions. It appeared that αs1-casein may be preferentially involved in these polymerizations with β-casein reacting at a somewhat slower rate. Polymerization and associated reactions modifying molecular charge led to the expected alterations in electrophoretic mobility and a loss of definition in the bands due to the various protein components. Measurements of proteolytic breakdown indicated only very small increases in trichloroacetic acidsoluble fragments, suggesting that proteolysis was of minor significance.