The objective of the study is to investigate the total antioxidative capacity of plasma and the level of survival of non-activated neutrophilic leucocytes in peripheral venous blood, their oxidative function and its changes when activated by non-opsonised E.coli and S.aureus, exposed to glucose and investigate the effect of glycosylated haemoglobin (HbA1c%) of different concentrations, using the methods of luminol- and liucigenin-dependent chemiluminescence, with respect to the patients suffering from type 1 diabetes mellitus (DM) and a severe form of periodontitis, as well as those who suffer from type 1 diabetes mellitus, but have healthy periodontal tissues, and also persons without systemic diseases and inflammatory periodontal diseases. An integrated investigation of the level of oxidative capacity of neutrophilic leucocytes in peripheral venous blood, using the methods of luminol- and liucigenin-dependent chemiluminescence, of the patients who suffer from type 1 diabetes mellitus and a severe form of periodontitis as well as persons suffering from type 1 diabetes mellitus, but with no inflammatory periodontal disease, and also healthy persons, has been performed for the first time. A new presumption has been presented on the basis of the collected material, which states that the development of inflammatory diseases of periodontal tissues is determined by the decreased level of survival of bactericidal myeloperoxidase system of neutrophilic leucocytes.