Gastric juice of lobster hydrolyzed the following substrates: triolein (enzymic activity: lipase), tributyrin ("lipase"), Azocoll(R) (proteinase), TAME and BAEE ("trypsin"), ATEE ("chymotrypsin"), HPLA ("carboxypeptidase A"), DNA (deoxyribonuclease), RNA (ribonuclease), p-nitrophenyl-phosphate (phosphatase), and p-nitrophenyl-N-acetyl-β-glucosaminide (chitobiase). (The quotation marks signify that the substrate specificities are typical of the quoted enzymes of mammalian or microbial origin. The spectra of specificities, however, of these enzymes and the corresponding enzymes of lobster are not necessarily identical.) Very low activities were found against RBB-starch (amylase) p-nitro-phenyl-α-(and β)-glucopyranoside(α- and β-glucosidase), p-nitrophenyl-β-galactopyranoside (β-galactosidase), and chitin azure (chitinase). No activities corresponding to phospholipase (lecithin), carboxypeptidase B (benzoylglycyl-L-arginine), elastase, glycylglycine dipeptidase, or leucine aminopeptidase were found.Gel filtration of gastric juice proteins on Sephadex(R) G-100 yielded estimates of molecular weights, among them: chitobiase 100,000; phosphatase 80,000; lipase 43,000; DNase 33,000; "trypsin" and ribonuclease 25,000; and two proteinases with optima around pH 4 and 8 and the low molecular weight of 12,500; proteolytic activity at pH 4 was also associated with the molecular weights 25,000 and 50,000.In ion-exchange chromatography, enzymes were eluted from DEAE-cellulose in the following sequence of increasing anionic character: chitobiase, proteinase (pH 4), proteinase (pH 8), lipase, "carboxypeptidase A," DNase, phosphatase, and "trypsin" and "chymotrypsin."Of the principal enzymes, only one (chitobiase) had its optimum at the pH 5 normal for lobster gastric juice. The lipase had an optimal pH around 7, phosphatase near 9, and "trypsin" near 8; proteolytic activity shows a maximum at pH 7–8 and another maximum, unusual for Crustacea, at pH 4.Much of the protein of gastric juice could not be connected with any enzymic activity. The proportion of nonactive protein seemed to increase with starvation of the animals. We found no evidence for the occurrence of zymogens.