The need to support food-labeling legislation has provided a driving force for development of analytical techniques for the analysis of food ingredients. The present work was carried out in view of a possible application in this field: the adulteration of milk and dairy products with milk from cheaper sources. The aim of this study was the development of a rapid and simple optical immunosensor, based on surface plasmon resonance (SPR) technology, for the detection of bovine IgG in adulterated milk samples and the detection and quantification of defined amounts of cow's milk that have been used to adulterate sheep and goat milk.