The inhibiting effect of rubella virus on lymphocyte stimulationin vitrowas studied, using purified virus of the attenuated strain RA 27/3. Addition of the virus to human lymphocytes from twenty healthy blood donors before stimulation with leuco‐agglutinin (LA), a component of phytohaemagglutinin, caused a considerable inhibition of the LA response in some experiments, whereas in other experiments the inhibition was slight or non‐existent. If further analysed, the results showed a correlation between the degree of inhibition and the immunity of the lymphocyte donor against rubella, as measured by haemagglutination inhibition (HI). Thus the LA‐response was significantly more depressed in a group of lymphocyte donors with HI‐titres ranging from 20 to 160 than in another group with HI‐titre less than 5. Possible explanations of the virus‐induced inhibition of the LA response and possible connection between this phenomenon and the immune response against rubella ar