The invention relates to an innovative modified sandwich-type immunoassay which uses polyclonal, monoclonal or recombinant antibodies against bovine immunoglobulins for the immunological detection and quantification of cow's milk in sheep's or goat's milk and, in particular, of cow's IgG in milk or cheese samples which is made from sheep's and/or goat's milk. More specifically, the invention provides procedures for the highest sensitivity and recovery quantification of bovine IgG in systems using polymers and disaccharides after the immobilization step of antibodies against bovine immunoglobulins and filter-sterilized standard solutions of known mixtures of bovine and sheep or goat milk. This innovative modified immune system identifies the adulteration of sheep or goat milk with cow's milk with excellent sensitivity and a high recovery rate, in a range of 0-12 percent.