Using an enzyme-linked immunosorbent assay, we measured the concentration of fibronectin containing an extra type III domain (ED1) in the circulation of humans. Plasma levels of ED1 + fibronectin averaged 2.8 +/- 1.0 micrograms/ml in healthy individuals and did not differ substantially according to age or sex. In comparison with those from normal subjects, plasma samples obtained from patients with collagen vascular disorders contained increased average levels of ED1 + fibronectin. Among this group, levels of ED1 + fibronectin were significantly greater in samples taken from individuals with clinical evidence of vasculitis. Although levels of total (ED1 + plus ED1 -) fibronectin were also elevated in plasma samples from patients with vasculitis, only the concentration of the ED1 + variant correlated with severity of disease in two patients examined serially. Elevations in plasma content of ED1 + fibronectin, but not total fibronectin, were also noted in patients with acute vascular tissue injury associated with major trauma or sepsis syndrome. Western blot examination revealed the presence of intact dimeric ED1 + fibronectin in the circulation of all patients studied, although fragments bearing the ED1 were also detected. Human plasma normally contains small quantities of soluble ED1 + ("cellular") fibronectin, and these levels are increased in disorders involving vascular injury.