1. Peripheral blood cultures from patients with infectious mononucleosis contained approximately 50 times more cells incorporating tritiated thymidine at time 0 than normal cultures.
2. Cultures from these patients showed active cell division within the first 24 hours. No cell division was seen during this time in cultures of normal cells.
3. Cells dividing within the first 24 hours were identified as cells which had incorporated thymidine at time 0 by observation of metaphases containing tritiated thymidine.
4. Cell division ceased after 18-24 hours in the absence of phytohemagglutinin.
5. The data suggest that a population composed of immature lymphocytic cells in the circulation of patients with infectious mononucleosis is in the process of cell division. Proliferative capacity of peripheral blood cells cannot be studied adequately by ordinary morphologic technic.