Bovine β-lactoglobulin (β-lg), α-lactalbumin (α-la) and bovine serum albumin (BSA), dispersed in ultrafiltration permeate, that had been prepared from whey protein concentrate solution (100 g/kg, pH 6·8), were heated at 75 °C. The consequent protein aggregation was studied by one-dimensional (1D) and two-dimensional (2D) polyacrylamide gel electrophoresis (PAGE). When 100 g β-lg/kg permeate solution was heated at 75 °C, cooled and examined, large aggregates were observed. These aggregates were partially dissociated in SDS solution to give monomers, disulphide-bonded dimers, trimers and larger aggregates. When mixtures of β-lg and α-la or BSA were heated, homopolymers of each protein as well as heteropolymers of these proteins were observed. These polymer species were also observed in a heated mixture of the three proteins. Two-dimensional PAGE of mixtures demonstrated that these polymers species contained disulphide-bonded dimers of β-lg, α-la and BSA, and 1:1 disulphide-bonded adducts of α-la and β-lg, or BSA. These results are consistent with a mechanism in which the free thiols of heat-treated β-lg or BSA catalyse the formation of a range of monomers, dimers and higher polymers of α-la. It is likely that when whey protein concentrate is heated under the present conditions, BSA forms disulphide-bonded strands ahead of β-lg and that α-la aggregation with β-lg and with itself is catalysed by the heat-induced unfolded BSA and β-lg.