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