The proteolytic enzymes in the visceral organs of albacore tuna fish (Thunnus alalunga) were studied, initially as an extract of the whole viscera, then as extracts of the individual organs. Preliminary studies indicated three pH optima of activity in the whole viscera extracts pH 1.7, 3.1-3.5 and 9.5. Analysis of extracts of the individual organs revealed the alkaline proteolytic activity was present in the intestine and the pyloric caeca, with the pyloric caeca showing the highest activity. The stomach extract demonstrated high protease activity at pH 1.7 and 3.5, while the remaining organs had relative activities at least ten times lower at these pH values. The extract obtained from pyloric caeca was studied in more detail. The approximate levels of trypsin-1ike and chymotrypsin-like activities were measured, and the effects of three inhibitors, phenylmethylsulfonylfluoride (PMSF), l-chloro-3-tosylamido-7-amino-L-2-heptanone (TLCK) and L-1-tosylamide-2-phenylethylchloromethylketone (TPCK), determined. Cationic discontinuous polyacrylamide gel electrophoresis coupled with a substrate inclusion technique for localization of the separated proteases of the pyloric caeca extract revealed the presence of ten proteolytic enzymes. One was active in acid solution (ca. pH 4) and was inhibited by PMSF and TLCK. The remaining nine proteases were active at pH 8.0. Five of the alkaline proteases were serine proteases and one of these showed trypsin-like specificity including PMSF and TLCK inhibition and activity against BANA. None of the electrophoretically separated proteases showed specificities similar to chymotrypsin. / Graduation date: 1976