Plasma levels of az-plasmin inhibitor (a2PI) and a,-plasmin inhibitor-plasmin complex (a2PlPC) were measured by sandwich enzyme-linked immunosorbent assay, using a recently developed monoclonal anti-a2PI antibody, in patients with collagen diseases. Twenty patients had systemic lupus erythematosus (SLE), 4 of whom also had vasculitis, 11 patients had rheumatoid arthritis (RA), 4 of whom also had vasculitis, and 5 patients had other types of vasculitis. There was no significant difference in azPl levels between the 3 patient groups and the control groups. However, plasma levels of a,PIPC in all 3 patient groups were significantly higher than those in the control group. Moreover, plasma concentrations of a2PIPC in SLE patients with vasculitis were statistically significantly higher than those in SLE patients without vasculitis. These concentrations were also higher in RA patients with vasculitis than in RA patients without vasculitis, although the difference was not statistically significant. Our findings indicate that measurement of plasma %PIPC levels is useful for detecting and evaluating the severity and activity of vasculitis in patients with collagen diseases.