A study of serum euglobulins carried out in 26 patients with diabetes mellitus and in 14 healthy persons permitted to detect definite differences between sick and healthy individuals. In the patients with diabetes mellitus (most distinctly in those with microangiopathies) the level of soluble and insoluble in the phosphate buffer euglobulins was greater than in the healthy persons. The microprecipitation reaction and immunoelectrophoresis with the monospecific immunoglobulins (IgA, IgM and IgG) more frequent and higher titers demonstrated IgA and IgG in diabetic patients. More pronounced heterogeneity (particularly distinct in the subfraction of euglobulins insoluble in the phosphate buffer) than in healthy individuals was revealed in the patients with diabetic microangiopathies by disc electrophoresis in polyacrylamide gel. The data obtained served as the basis for the search in the serum euglobulins of the patients with diabetes of compounds possibly possessing the antigenic properties, capable of producing antibodies and immune complexes, i.e. the immunological factors taking part in the vascular wall injury.