An enzyme-linked immunosorbent assay (ELISA) has been developed for the quantification of alpha 1-antitrypsin-human leukocyte elastase (alpha 1AT-E) complexes. In the ELISA, the alpha 1AT-E complex is bound to a surface by rabbit antileukocyte elastase antibody, and the inhibitor- proteinase complex is quantified by a second antibody, rabbit anti- alpha 1-antitrypsin F(ab')2, labeled with alkaline phosphatase. alpha 1AT-E complexes were detected when a final concentration of 2.2 nmol/liter of leukocyte elastase was added to plasma. The concentration of these complexes increased with additional elastase. In clotting blood, alpha 1AT-E complexes were generated in parallel with the conversion of 125I-fibrinogen to fibrin, whereas alpha 2-plasmin inhibitor-plasmin (alpha 2PI-P) complexes were not formed. The concentration of alpha 1AT-E complexes in 19 of 21 controls was less than 2.2 nmol/liter. Patients with laboratory evidence for disseminated intravascular coagulation (DIC) demonstrated elevated alpha 2PI-P complexes with either increased or normal concentrations of alpha 1AT-E complexes. Patients without evidence for DIC, but who demonstrated prolonged reptilase clotting times, were studied. This group had increased alpha 1AT-E but normal alpha 2PI-P complex levels, raising the possibility that elastase release in vivo may be accompanied by limited degradation of fibrinogen. These assays thus serve as useful probes for the study of leukocyte activation and of the interactions between cellular and plasma proteolytic enzyme systems.