The phytohaemagglutinin (PHA) response of lymphocytes from seven patients with natural rubella infection was investigated during the acute, early and late convalescence stages of disease. When the lymphocytes were cultured in autologous serum, a moderate depression of the response, following stimulation with PHA in both optimal and suboptimal concentrations was obtained two weeks after the onset of exanthema (early convalescence). Two months later (late convalescence), the lymphocyte stimulation response had returned to almost normal values. On the other hand, lymphocytes incubated in pooled homologous serum reacted normally to PHA in optimal concentrations at all three stages. Determination of T- and B-lymphocytes did not reveal any change in the relative proportion of T-lymphocytes during the course of the disease. However, in late convalescence, a significant decrease in the relative number of B-lymphocytes was recorded.