Protease inhibitory activity in jackfruit seed (Artocarpus integrifolia) could be separated into 5 fractions by chromatography on DEAE-cellulose at pH 7.6. A minor fraction (I) that did not bind to the matrix, had antitryptic, antichymotryptic and antielastase activity in the ratio 24:1.9:1.0. Fraction II bound least tightly to the ion exchanger eluting with 0.05 M NaCl and could be resolved into an elastase/chymotrypsin inhibitor and a chymotrypsin/trypsin inhibitor by chromatography on either immobilized trypsin or phenyl Sepharose CL-4B. Fractions III and IV eluted successively with 0.10 M NaCl and 0.15 M NaCl from DEAE-cellulose, inhibited elastase, chymotrypsin and trypsin in the ratio 1.0: 0.53:0.55 and 1.0:8.9:9.8 respectively. Fraction V, most strongly bound to the matrix eluting with 0.3 M NaCl and was a trypsin/chymotrypsin inhibitor accounting for 74% of total antitryptic activity. This inhibitor was purified further. The inhibitor with a molecular weight of 26 kd was found to be a glycoprotein. Galactose, glucose, mannose, fucose, xylose, glucosamine and uronic acid were identified as constitutent units of the inhibitor. Dansylation and electrophoresis in the presence of mercaptoethanol indicated that the inhibitor is made up of more than one polypeptide chain. The inhibitor combined with bovine trypsin and bovine α-chymotrypsin in a stoichiometric manner as indicated by gel chromatography. It had very poor action on subtilisin BPN′, porcine elastase, pronase,Streptomyces caespitosus protease andAspergillus oryzae protease. It powerfully inhibited the caseinolytic activities of rabbit and horse pancreatic preparations and was least effective on human and pig pancreatic extracts. Modification of amino groups, guanido groups and sulphydryl groups of the inhibitor resulted in loss of inhibitory activity. Reduction of disulphide bridges, reduction with sodium borohydride and periodate oxidation also decreased the inhibitory activity.