Two groups of young rubella-susceptible women were vaccinated with two rubella vaccines. Heparinized blood samples were taken from all individuals the day of vaccination and 5, 7, 15, 21, 30, 35, and 42 days later. Purified lymphocytes from these samples were cocultivated with AGMK cells for rubella virus isolation. Parallel samples of lymphocytes were stimulated with phytohemagglutinin, and the rate of [14C]thymidine incorporation was determined. Rubella virus was isolated from lymphocytes collected on days 7, 15, and 21 after RA27/3 vaccination in contrast to days 7 to 35 after HPV77 vaccination. The lymphocyte response to phytohemagglutinin was markedly suppressed from day 5 to 15. Normal lymphocyte responses were restored within 1 month after vaccination with RA27/3, but even later (1 week) after HPV77 vaccine. Lymphocytes from rubella-susceptible persons infected invitro with rubella virus vaccines and stimulated with phytohemagglutin displayed a decrease in their responsiveness to the mitogen similar to that observed with lymphocytes from vaccinees. The transient immunosuppression observed in vaccinees is probably due to virus-induced functional damage of the lymphocytes since no direct cytocidal effect of rubella vaccine has been demonstrated on human lymphocytes.