Infectious mononucleosis in the past has signified to many physicians a mild infectious disease primarily affecting the nasopharynx and distinguishable from ordinary infections of the upper respiratory passages by striking enlargement of the lymph nodes, the appearance in the blood of large numbers of lymphocytes of characteristic bizarre type, and the development during convalescence of heterophile agglutinins for sheep red blood cells. It was early recognized, however, that many cases differ markedly in their clinical features from this classical type and that in individual cases there may be evidence of involvement of many different organs and tissues. Among these may