The effect of anti-rubella vaccination on lymphocyte proliferation kinetics in vitro and on SCE frequency was studied in three young women. The studies were carried out by taking blood samples before vaccination (day 0) and subsequently on days 7, 14, 28 and 42. The mitotic index (MI) shows a decrease at day 14 and 28 followed at day 42 by an increase above day 0 levels. The average cell cycle (ACC) shows a decrease at day 14 followed by an increase at day 28. Complex variations were also found in the percentage distribution of cells in the various division classes. The SCE frequency showed variations inverse to the MI. The whole picture seems to indicate the existence of changes in the lymphocyte population which can be correlated with immune response.