Human .alpha..sub.2-antiplasmin (.alpha..sub.2AP) is the major inhibitor of the proteolytic enzyme plasmin that digests fibrin. Two forms of .alpha..sub.2AP circulate in human plasma: a 464-residue protein, which we have termed "pro"-form, or .alpha..sub.2AP.sub.pro, and an N-terminally-shortened 452-residue "activated"-form, or .alpha..sub.2AP.sub.act. The latter becomes crosslinked to fibrin by activated factor XIII about 5-fold more rapidly than .alpha..sub.2AP.sub.pro and makes fibrin resistant to digestion by plasmin. A new human plasma proteinase has been identified herein that cleaves the Pro12-Asn13 bond of .alpha..sub.2AP.sub.pro to yield .alpha..sub.2AP.sub.act. This enzyme is identified herein as Antiplasmin Cleaving Enzyme (APCE).