Membrane TNF-alpha is a precursor form of soluble TNF-alpha and exerts pro-inflammatory functions in a cell-to-cell contact manner. We showed that membrane TNF-alpha is induced upon activation on the cell surface of CD4+ T cells and CD8+ T cells. In patients with systemic lupus erythematosus (SLE), the percentage of membrane TNF-alpha-bearing CD8+ T cells (41.5+/-12.3%) was significantly higher compared with those of healthy controls (26.7+/-3.9%) (p=0.007) or patients with rheumatoid arthritis (29.8+/-15.4%) (p=0.038). Membrane TNF-alpha-bearing CD8+ T cells from SLE patients displayed cytotoxic activity against L929 cells. It is possible that membrane TNF-alpha may be involved in the increased apoptosis and the generation of autoantigens in SLE.