Polyclonal B cell activation is the most visible biological manifestation of systemic lupus erythematosus (SLE) autoimmunity. Murine models and in vitro lymphocyte studies are the most important tools used to improve our comprehension of the disease. It was successively demonstrated that there is an intrinsic B lymphocyte hyperreactivity in human and murine lupus; that the B lymphocytes overreact to stimulating factors produced by T lymphocytes; and that these stimulating factors could be over-produced. This last feature contrasts with decreased interleukin 2 production and lymphocyte response to this cytokine. A more precise study of the interleukins involved in the control of the humoral response shows the importance of interleukins 4, 5, 6 and of gamma-interferon. Further investigations are needed to improve our understanding of B cell hyperreactivity during SLE. These studies will benefit from better molecular characterization of many interleukins and their receptors.