Increased 3H-Thymidine(3H-TdR) uptake was noticed when peripheral blood mononuclear cells (MNC) from systemic lupus erythematosus(SLE) patients were cultured without any mitogenic stimulation. The peak of 3H-TdR uptake was found immediately after transfering the cells to in vitro culture. DNA synthesizing cells were observed directly by means of autoradiography by pulsing with 3H-TdR for 4 hours. Therefore, it was presumed that there were some MNC in the peripheral blood from SLE patients which were already triggered to proliferate in vivo. These spontaneously proliferating MNC were found to fractionate into a nonadherent, non-E-rosette forming and surface immunoglobulin negative cell population, the so called null cell population. These results suggest that there were activated MNC in the peripheral blood from patients with SLE which fractionated into a null cell population.