3,4 Dichloroisocoumarin is a potent, irreversible inhibitor of serine proteases. It reacts with serine proteases to release an acylchloride moiety that can acylate another active site residue. 3,4 Dichloroisocoumarin also inhibits granzymes A, B, and H, cathepsin G, neutrophil elastase, proteinase and blocks apoptotic internucleosomal DNA cleavage in thymocytes without the involvement of endonucleases. 3,4 Dichloroisocoumarin inhibits activation of neutral sphingomyelinase and daunorubicin-triggered apoptosis as well as apoptosis induced by camptothecin in HL-60 cells. 3,4 Dichloroisocoumarin inhibits migration of eosinophils through basement membrane components in vitro. 3,4 Dichloroisocoumarin does not affect thiol proteases and metalloproteases. 3,4 Dichloroisocoumarin does not exhibit any activity towards β-lactamases. 3,4 Dichloroisocoumarin is an inhibitor of Chymotrypsin and Trypsin.