Complex formation in vitro between human alpha2-macroglobulin and the human proteases cationic trypsin, chymotrypsin, plasmin and granulocyte elastase and collagenase was clearly visualized by the use of thin-layer electrofocusing in polyacrylamide gel followed by electrophoresis in agarose gel containing antibodies against human alpha2-macroglobulin. The technique permits semi-quantitative determination of the amount of complex and can demonstrate the formation of complexes between alpha2-macroglobulin and protease in vivo in ascitic fluid in peritonitis.