T cells from 18 untreated SLE patients produced significantly more B cell growth factor (BCGF) than did those from normal subjects. Those from SLE patients with active disease produced significantly more than did those from patients with inactive disease. The response to BCGF of SAC-stimulated B lymphocytes from SLE patients was higher than that of B lymphocytes from normal individuals. Similarly preactivated B cells from five of seven SLE patients also proliferated upon the addition of interleukin 1 (IL-1) whereas those of normal subjects did not. Simultaneous addition of IL-1 and BCGF had a synergistic proliferative effect on B cells from two of seven SLE patients but not on any of the controls. Interleukin 2 (IL-2) had no proliferative effect in either SLE or normal B cells. Supernatant fractions from T cells of seven of 10 patients with active SLE and three of 10 with inactive SLE induced more IgG production by CESS cells than did those of normal subjects indicating a higher production of B cell differentiation factor by SLE T cells than by those of controls. Our findings may explain the reported preactivation and predifferentiation of peripheral blood B cells from SLE patients and give insight into the mechanisms leading to the production of autoantibodies in this disease.