(1). Leukocytes from eight patients with infectious mononucleosis, one with chronic lymphocytic leukemia, and from three normal individuals were examined with the light and the electron microscope.

(2). Most of the mononuclear cells in infectious mononucleosis belonged to the lymphocytic series.

(3). Mitochondria in lymphocytes from patients with infectious mononucleosis were generally larger and more varied in appearance than those from normal individuals and from the patient with leukemia.

(4). Although there was some variation in the density of the cytoplasm, number of PNA granules, and appearance of the mitochondria, the differences were insufficient to justify an electron microscopic classification of lymphocytes as atypical or typical, as is possible with Wright-stained smears in light microscopy.

(5). Different forms of granules and inclusions in leukocytes were shown and their possible significance discussed.

(6). Electron dense deposits in channels interpreted as endoplasmic reticulum were demonstrated in some neutrophils.

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