The in vitro DNA and RNA synthesis of infectious mononucleosis (I.M.) cells have been studied in four patients. For evaluation, monocytoid, lymphocytoid and plasmocytoid I.M. cells were distinguished. Short‐term incubation with H3‐thymidine revealed that the DNA synthesis index was highest in plasmocytoid forms with 21 per cent labelling, followed by monocytoid I.M. cells with 7.5 per cent and lymphocytoid I.M. cells with 1.3 per cent uptake. In vitro RNA synthesis was also characterized by most active rates of incorporation of H3‐uridine and H3‐cytidine in plasmocytoid cell types and a similar drop in the intensity of uptake for the other two forms. The still enigmatic origin and function of these irritation cells of the peripheral blood are emphasized. The rather striking autoradiographic findings of cellular proliferation and active RNA synthesis in these cells reveals properties of immature cells. The previously suggested possibility of intracellular viral synthesis is considered to be rather unlikely from our findings. On the other hand, a relationship to multipotent mononuclear stem cells which today have to be assumed to occur in the peripheral blood of even healthy individuals, is di