Various changes in the granulocytes have been investigated in a large series of patients with infectious mononucleosis. A high proportion of cases regularly show a neutropenia with a shift to the left during the first three to four weeks of the disease; these changes, though transient, may be profound. The band cells present during the acute phase of the disease also show certain alterations in cytochemical staining and low alkaline phosphatase scores are common at this time. The maturation and release of leucocytes from the bone marrow is apparently normal. Theoretical mechanisms for the neutropenia are briefly discussed, including the possible role of excessive peripheral destruction.