Systemic lupus erythematosus (SLE) is a disease of immune dysregulation in which B cell hyperactivity and T cell deficiency are important characteristics. Sex factors also play a major role in the pathogenesis based on the physiologic effects of estrogen in promoting immunologic hyperactivity. Our findings suggest that a posttranscriptional mechanism is responsible for the functional interleukin 2 (IL-2) defect since transcription of the IL-2 message occurs after mitogenic stimulation. The proliferating cell in the MRL/lpr mouse model of lupus may be an immature T cell. The T cell receptor in these mice has a lower molecular weight than normal. This aberrant T cell receptor might be explained by a defect in glycosylation. The administration of estrogen to pregnant mice late in gestation results in offspring with a permanently altered immune system. These mice develop features of autoimmunity similar to those that occur spontaneously in genetically susceptible autoimmune mice. This phenomenon may have etiopathological significance for familial SLE.