In order to study the reaction behavior of bovine holo- and apo-$alpha$-lactalbumin ($alpha$-La) during heat treatment at 65~10$0^{circ}C$, the samples were analysed by first (ID)-and second-dimensional (2D) native-polyacrylamide gel electrophoresis (Native-PAGE) and sodium dodecylsulfate (SDS)-PAGE. When bolo-$alpha$-La or apo- $alpha$ -La were heated, they formed non-native, monomers, dimers and trimers. The apo-$alpha$-La was more heat-sensitive than holo-$alpha$-La. The monomers seemed to have the same composition as the native $alpha$-La, but many of the disulfide bonds could be non-native.