The “ Infectious Mononucleosis Cell

iii infectious mononucleosis has been the subject of controversy. It is now generally accepted that they belong to the lymphocytic series 3 although an occasional worker4 holds out for their monocytic origin. Recently, Paegle,#{176} usmg the electron microscope, found that most of the cells in the blood of l) ttients with infectious mononucleosis contained organelles similar to those of the cells of the lymphocytic series and did not have an ergastoplasm. He vas unable to distinguish between the three cell types described by Downey.2 It is the purpose of this study to characterize the infectious mononucleosis ( IM ) cells cytochemically in an attempt to throw further light on their origin, nature and function. The study was restricted largely to the atypical lymphocytes as they appeared in the peripheral circulation, although imprint preparations of hiopsied lymph nodes were investigated in two cases.