Human platelets contain a pool of C1 inhibitor (C1 INH) distinct from that in plasma. Twelve normal platelet samples washed by centrifugation had a mean platelet C1 INH antigen level of 19.3 +/- 2.8 ng (mean +/- SEM) per 10(8) platelets. These values contrast with the mean +/- SEM platelet C1 INH antigen level of 6.1 +/- 0.9 per 10(8) platelets from 12 C1 INH-deficient patients. The level of platelet C1 INH correlated (r = .7) with the level of plasma C1 INH in normal individuals and patients with classic hereditary angioedema. Platelet C1 INH, like plasma C1 INH, was a 105-Kd protein on immunoblots of solubilized platelets and in thrombin- or collagen-induced platelet releasates. On indirect immunofluorescence, morphologically and immunochemically identifiable elutriated human megakaryocytes had C1 INH antigen. Using nested primer polymerase chain reaction, C1 INH mRNA was detected in megakaryocytes. When activated, human platelets expressed a portion of their total pool of C1 INH antigen on their membrane. Using a competitive enzyme-linked immunosorbent assay for C1 INH as a quantitative, indirect antibody consumption assay, the surface of unstimulated platelets had 0.55 +/- 0.4 ng C1 INH/10(8) platelets (mean +/- SEM). When activated with thrombin, platelets secreted 7.37 +/- 2.2 ng C1 INH/10(8) platelets into the suspension buffer and simultaneously expressed 4.4 +/- 1.2 ng C1 INH/10(8) platelets on their external membrane. These studies showed that activated platelets secreted 38% of their C1 INH and externalized another 23% of the total platelet C1 INH on their membrane. Furthermore, in 125I-anti-C1 INH Fab' binding experiments to platelets, about 8 ng of the antibody fragment specifically bound to 10(8) activated platelets. These data suggest that level of platelet C1 INH packaged into platelet alpha-granules is modulated by the amount of protein produced in megakaryocytes. Platelet alpha-granule C1 INH can both be secreted from platelets and expressed on their activated membranes. The cell membrane expression of C1 INH may be important to modulate the activity of the proteases of the complement and contact systems of plasma proteolysis in the microenvironment of the inflammatory response.