The incidence of positive heterophil tests in infectious mononucleosis is a much disputed point. The present day consensus in regard to this question can be epitomized. as follows: A minor but substantial percentage of patients with infectious mononucleosis have diagnostic blood smears but negative heterophil reactions; therefore, a positive heterophil reaction is a desirable adjunct but not essential to the diagnosis. It is often difficult to ascertain just how serious an attempt has been made to diagnose these cases serologically. If these seronegative cases are classified on the basis of a single negative reaction obtained during the first seven days of the illness, the results are understandable. In my experience all patients with infectious mononucleosis characterized by a positive blood smear have shown a diagnostic titer of heterophil antibodies, provided the blood was tested serially through the second week of the disease. MATERIAL This study is based on observations on