Lymphocyte phytohemagglutinin (PHA) responsiveness was found suppressed in both rubella sero-negative and sero-positive recipients of RA 27/3 strain of live attenuated rubella vaccine; the suppression was readily demonstrable only when a suboptimal dose of PHA was applied in the test. Lymphocytes from sero-negative vaccinees, which initially showed little or no in vitro response to concentrated rubella virus, became responsive after vaccination by day 21, when the highest sensitization to rubella antigen was seen. In the sero-positive vaccinees. lymphocytes responded to rubella antigen in vitro before vaccination, and in most cases vaccination did not result in significant changes in lymphocyte response. These results suggest that rubella vaccination leads to temporarily increased lymphocyte reactivity to rubella antigen, and the increased lymphocyte response to specific antigen may occur at the time of mild suppression of PHA response.