AbstractThe relationship between inter‐α inhibitor (IαI) and urinary proteinase inhibitor (UPI) was examined by comparing purified UPI with a proteolytic fragment of IαI (l'), and by demonstrating that inflammatory cells produce similar fragments under physiologic conditions. Purified I', derived by chymotrypsin digestion of IαI, was similar to UPI in apparent molecular weight (68,000‐69,000), amino acid composition, immunoreactivity, and inhibitory activity against trypsin, chymotrypsin, and neutrophil elastase. The production of similar inhibitory fragments by murine peritoneal macrophages, human neutrophils, and a murine mast cell line was quantified. Neutrophils were most efficient at proteolyzing IαI. Comparison of the pattern of IαI degradation by neutrophil preparations with that by pure enzymes, suggested that both elastase and cathepsin G mediate neutrophil proteolysis of IαI. These proteinases may thus be responsible for inflammation‐related increases in UPl‐like inhibitor levels in vivo.