1. The findings in the blood and in aspirated bone marrow in 23 cases of infectious mononucleosis have been described.

2. Unequivocal evidence of involvement of the bone marrow has been found in 70 per cent of the cases.

3. Evidence of granulomatous inflammation of the marrow was found in 48 per cent of the cases.

4. Epithelioid cells were found in the films of bone marrow in 48 per cent of the cases. These cells appear morphologically identical with those seen in imprints of lymph nodes from infectious mononucleosis and sarcoidosis and with the epithelioid cells seen in films of the marrow in brucellosis, sarcoidosis and tuberculosis.

5. The granulomatous lesions of infectious mononucleosis seem most similar to those of brucellosis, but they also resemble the small granulomatous lesions of sarcoidosis and tuberculosis.

6. Lymphocytosis of the marrow as well as of the blood was demonstrated in all cases. Evidence of formation of lymphocytes in the marrow was presented, and the altered lymphocytes of infectious mononucleosis were found in films of the marrow. The degree of lymphocytosis of the marrow in infectious mononucleosis was shown to be less than that in lymphatic leukemia. Lymphocytosis of the marrow was not found in brucellosis, sarcoidosis or tuberculosis. The lymphocytic reaction demonstrable in the marrow in infectious mononucleosis is believed to be of value in differential diagnosis.

ACKNOWLEDGMENTS We wish to acknowledge the generous cooperation of Dr. Ruth E. Boynton and the Staff of the Student’s Health Service of the University of Minnesota throughout the course of this year long study of infectious mononucleosis. We are indebted to Dr. T. Edward Bell and Dr. James Cardy for performing the sternal aspirations. The photomicrographs were made by Mr. Henry Morris.