A trypsin/chymotrypsin inhibitor was isolated from the tubers of Alocasia macrorhiza by extraction at pH 7.6, heat treatment at 80°C, ammonium sulphate precipitation and successive column chromatography on CM-cellulose, DEAE-Sephadex A-50 and Sephadex G-100. The inhibitor was pure by cellulose acetate electrophoresis. The molecular weight was approximately 32 000 as determined by gel filtration on Sephadex G-100. The inhibitor acted on bovine trypsin, human trypsin and bovine chymotrypsin. It had no action on human chymotrypsin, subtilisin BPN, pronase, Aspergillus oryzae protease, human and porcine pepsins. The bindings sites for bovine trypsin and chymotrypsin are not mutually exclusive. The inhibitor was stable over a pH range of 1-10. The purified inhibitor was far more thermostable than the crude inhibitor. The purified inhibitor lost only 33% of activity on heat treatment at 95°C for 2 h. Trinitrobenzene sulphonate treatment resulted in the loss of antichymotryptic activity faster than the antitrypsin activity of the inhibitor. © 1977.