It has been suggested that kallikrein inhibition may predispose patients with the lupus inhibitor to thrombosis by interfering with the Factor XII-mediated activation of plasminogen. To further investigate this suggestion, the authors measured kallikrein inhibition in 19 patients with lupus inhibitor. They found that kallikrein inhibition was greater than 100% of that of a normal plasma pool in all patients with greater than 125% in 11 of 19. Kallikrein inhibition was significantly correlated with C1-esterase inhibitor (C1-s-INH) concentration, which they measured by rocket immunoelectrophoresis (r = +0.55, P < 0.05). In three patients the C1s-INH was more than 30% greater than the kallikrein inhibition. Crossed immunoelectrophoresis for C1s-INH in these patients' plasma revealed an electrophoretic mobility identical with that of the normal plasma pool. The authors suggest that c1s-INH-mediated kallikrein inhibition, in conjunction with other coagulation abnormalities, predisposes patients with the lupus inhibitor to thrombosis.