The acid-stable trypsin inhibitor of human serum and urine is released in vivo by limited proteolysis from the high molecular weight, acid-labile inter-alpha-trypsin inhibitor. When complexed with trypsin, both this acid-stable, active derivative and the inter-alpha-trypsin inhibitor can be degraded in vitro by prolonged digestion with trypsin to a low molecular weight "minimal" inhibitor. This minimal trypsin inhibitor was sequenced and found to be homologous to the known Kunitz-type inhibitors (e.g. the basic trypsin-kallikrein inhibitor from bovine organs). This indicates that the antitryptic activity of the big inter-alpha-trypsin inhibitor is due to a Kunitz-type domain.