Systemic lupus erythematosus (SLE) in humans and in mice appears to be a syndrome with different cellular bases. In individuals, the illness is influenced by a variety of factors, including genetic, hormonal, immune, and environmental. In mice, SLE can be induced with polyclonal B-cell activators and thymectomy. Retardation of disease occurs with the gene xid on an NZB background. Tolerance studies indicate that tolerance depends upon a normal thymus. In addition, females can be nontolerant with a single immune defect, whereas males can become tolerant with only one defect; they become nontolerant with two immune defects. These studied may help to explain the prevalence of SLE in females and the protective effects of androgens. Human SLE is characterized by excessive B cell activity and impaired T cell activity, especially in active disease. A scheme by which the disease becomes activated is put forth. The details of cell-cell dialogue are becoming clearer with study of the autologous mixed lymphocyte reaction. T 4+ cells provide helper signals for T 8+ cells. Macrophages and T cells combine to regulate the AMLR. The AMLR itself gives rise to a variety of functional cells and serves as an amplification system. This system is defective in active SLE. Preliminary attempts have been made to separate human SLE into subgroups on the basis of the ratio of helper to suppressor cells (RT). A low RT is associated with renal disease, whereas a high RT characterizes patients with a multisystem illness with less important kidney involvement.