The serum of 11 cases of severe diabetic acidosis were tested for insulin-activity and for insulin-inhibiting potency with the aid of the rat-diaphragm in vitro test. In 3 cases no insulin activity could be detected, and in the others insulin-activity was low. In 4 cases the serum exhibited insulin-inhibiting activity. In 2 of these 4 cases the serum was subjected to column electrophoresis without anti-insulin-activity being found in the fractions. The sera of 4 labile diabetic patients on insulin therapy without severe acidosis, and of 6 diabetic patients without severe acidosis who never received insulin or oral anti-diabetic drugs, were also examined. The sera of all these patients showed essentially normal insulin-activity and no detectable anti-insulin-activity. After column electrophoresis (5 cases) the insulin-activity of these non-coma sera could be localized in the β- and γ-globulin fractions.