CD28 is a 44-kDa glycoprotein that contributes to T cell activation and proliferation. To elucidate the functional role of CD28 in T cell proliferation and IL-2 production in SLE, we studied its effects in cells from untreated patients with active (n = 10) or inactive disease (n = 10) as compared with normal subjects. Mean percentages of CD4+ CD28+ and CD8+ CD28+ T cells were decreased in SLE patients (P < 0.01). SLE patients had significantly decreased absolute CD8+ CD28+ T cells. To investigate whether CD28 antibody affects T cell proliferation, we stimulated peripheral blood T cells from SLE patients and normal controls with anti-CD28, anti-CD3 and/or Interleukin-2 (IL-2) during 3 days of culture. We found that T cells from SLE patients had significantly higher responses to CD28 than did cells from normal controls. This effect was higher in cells from patients with active disease than in those with inactive disease. Conversely, IL-2 had no significant effect on the proliferative response of SLE T cells. However, when it was used for co-stimulating with anti-CD28, there was an increase in the secretion of IL-2 which was greater in the cells of patients with active disease. Thus, on average, there was an 81% increase in the production of IL-2 in T cells from patients with active SLE, 48% in those from patients with inactive disease and 40% in T cells from healthy controls, as compared with the production in response to stimulus by anti-CD3 or with anti-CD3 and anti-CD28. Lymphocytes from patients with active disease showed increased gene expression of CD28 when compared with normal subjects. These data suggest that CD28 might play a central role in the defective immune response observed in SLE patients.