Cytologic and serologic studies of the blood of persons with "glandular fever" have led to a renewed interest in the condition. The demonstration of the marked increase of mononuclear cells, which has caused the general use of the term infectious mononucleosis, and the recent discovery by Paul and Bunnell1 of a high concentration of sheep cell agglutinins in the blood of patients with the disease indicate that it is quite different from most infectious processes. Its importance, however, as a subject of investigation is far greater than that of a mere hematologic curiosity for the following reasons: First, it is a rather common and benign condition, an accurate diagnosis of which allows prompt reassurance of the patient and a fairly accurate prognosis as to duration and morbidity. Second, the differential diagnosis from more serious conditions, especially lymphatic leukemia, is of great importance. Third, a solution of the problem involved