When an isolated mouse diaphragm was incubated with an insulin-like activity-possessing protease (ILAPP), the amount of Folin-positive materials released into medium increased as the incubation proceeded. However, the digestion by ILAPP did not affect the action of epinephrine on the glycogen content in diaphragm. Comparative studies on the effects of ILAPP and insulin on the glycogen content in diaphragm incubated in the presence or absence of ouabain at different concentrations of K+ increased in the incubation medium, that the glycogen-increasing effect of ILAPP or insulin was observed in each buffer containing K+ at a concentration of 0, 0.59, or 5.9 mM, and that the additive effect of ouabain used together with ILAPP or insulin was distinctly observed in the 5.9 mM K+-buffer but not in the 0.59 mM-buffer or K+-buffer. Thus, the effects of ILAPP resembled those of insulin. ILAPP probably digests the surface of diaphragm at a limited portion other than an epinephrine receptor to exhibit insulin-like activity.