Two recent techniques using optical immunosensor technology were developed for the quantification of milk proteins in dairy products. The first application is the simultaneous quantification of the 3 major caseins (alpha s(1), beta, and kappa). This assay consists of a 2-step sandwich strategy, with 2 monoclonal antibodies directed against the N- and C-terminal extremities of each of the caseins, respectively. This strategy permits only intact caseins to be quantified, and not their degradation products. The technique is fast (10 min), sensitive (detection limit about 0.87 mu g/mL), and has been applied successfully to raw and drinking milks. In the second application, the severity of the heat treatment sustained by a milk of unknown origin is determined by quantifying separately the native and heat-denatured forms of alpha-lactalbumin with specific monoclonal antibodies. The technique allows discrimination of the different heat treatments studied (pasteurization, direct and indirect ultra-high temperature, sterilization), is fast (4 min), repeatable, fully automated, and requires no pretreatment of the milk sample.