An enzyme-linked immunoassay detecting soluble CD8 (s-CD8) was applied to study activation of CD8(+)-(suppressor/cytotoxic) T-cells in patients with rheumatic diseases. Compared with normals, s-CD8 levels were elevated in patients with rheumatoid arthritis, ankylosing spondylitis, and polymyositis. In contrast, low s-CD8 values were observed in patients with progressive systemic sclerosis/scleroderma. In systemic lupus erythematosus (SLE), s-CD8 values were correlated with C-reactive protein. This finding and an association with other parameters of clinical activity were confirmed by longitudinal studies. In summary, our findings support the view that implication of CD8(+)-T-cell activation is different in the pathogenesis of each rheumatic disease. Elevated s-CD8 indicates active disease, and can be used to monitor CD8(+)-T-cell activation in SLE while determination of s-CD8 seems to be of little clinical value in the other rheumatic diseases studied.