9,249 persons were examined by Paul-Bunnell-Davidson test and in 329 heterophile antibodies were established in titre > 1/40 which enables, with corresponding clinical-laboratory parameters, diagnosis of infectious mononucleosis caused by Epstein-Barr virus. In 271 persons diagnosis could not be established by testing only one sample of serum and performing only one test. That is why Epstein-Barr virus specific tests were recommended and their diagnostic possibilities in certain stages of Epstein-Barr virus infection have been theoretically considered.