Infectious Mononucleosis (IM) is an acute pathology caused by Epstein- Barr Virus (EBV). The most frequent symptoms are fatigue, pharyngitis, fever, linphoadenopatia, splenomegaly and hepatitis. Heterophyl antibodies are the primary antibodies which appear in the patient serum, usually from one to three weeks after the beginning of the symptoms and in 85-95% of the cases. These antibodies are principally represented by IgM class, and may be detected the in patient serum more than a year after the disease arising. A reliable IgM diagnosis can be done based on the heterophyl antibody determination. These antibodies can react with the membrane antigens of the erythrocytes infected by several mammals species. The test is based on the immunological reaction of the heterophyl antibodies associated to mononucleosis and to sensitized latex particles. These particles are stabilized in order to avoid any cross reaction with Forssman heterophyl antibodies. The agglutinate is easily visible with the naked eye.