1. Electrophoretic separation of proteases from human gastric mucosal extracts of five patients with gastric ulcer, one with duodenal ulcer and three with gastric cancer were investigated by agar-gel electrophoresis at pH 8.3 and pH 5.0. 2. In the fundic mucosal study, there were seven faster moving proteases in all nine cases, but the slowest moving protease showed a slightly different picture in each case. In the antral mucosal study, two of eight cases showed mainly group II pepsinogens, seven of nine cases, however, showed the same results as in the fundic mucosal study. 3. In the cases of the nine mucosal extracts activated at pH 1.5 or pH 4.0, they all showed the same electrophoretic separation at each pH level. At these two pH levels, however, quite different electrophoretic patterns were observed. The presence of pepsin 3 appeared to diminish at the higher levels of pH, although that of pepsin 5 and pepsin 7 appeared to increase at pH 4.0 and above. Pepsin 6 appeared for the first time at pH 4.0 and existed at higher pH levels. 4. We thus conclude that electrophoretic patterns of pepsins in the gastric mucosal extracts are changeable depending on the pH level of the incubating medium, and further that diversity of pepsins in gastric juices may also depend on the pH level of gastric juices.