Cytophotometric determinations of DNA were carried out with a pulse cytophotometer in peripheral blood leucocytes in 25 patients with infectious mononucleosis and 30 patients with other infectious diseases. DNA cytograms of patients with mononucleosis showed presence of a high percent of cells in phase S and G2+M of the cell cycle. This is an evidence of active proliferation of these cells persisting for about 6--8 weeks. The authors believe that pulse cytophotometry may be used for detection and investigation of states with lymphocytes stimulation in vivo, e.g. in certain infectious diseases, during immunotherapy of neoplasms, rejection reaction etc.