In 24 patients with systemic lupus erythematosus the proliferating response of lymphocytes of the peripheral blood stream to mitogens PHA, ConA and PWM was examined. The spontaneous incorporation of 3H-thymidine was increased in two patients. The mean response to PHA and PWM was reduced, while the response to ConA was within the normal range. In the active stage of the disease the lymphocyte response was in general lower in the stage of low activity. There was no correlation between the number of CD4 and CD8 lymphocytes nor the CD4/CD8 index and the lymphocyte response to mitogens. In systemic lupus erythematosus the response to PWM was lower than in rheumatoid arthritis, while in rheumatoid arthritis the response to PHA and ConA was lower than in systemic lupus erythematosus.