Objective — to comparatively investigate the characteristics of the oxidative-antioxidant system in early preterm labor, its threat, in early preterm labor in the presence of early amniorrhea and a positive C-reactive protein level in early amniorrhea as an acute-phase marker of inflammation. Subject and methods. Examinations were made in 120 pregnant and parturient women at 28—33 weeks’ gestation, among them there were 80 patients with threatened early preterm labor (n=20) and its different clinical types [incipient preterm labor without early amniorrhea (n=20), that developing 4—7 days after amniorrhea (n=20), and that occurring 4—7 days after amniorrhea with a positive C-reactive protein level of (n=20), these were 60 patients], and 20 pregnant women with physiological pregnancy who formed a control group. A comparison group included 20 apparently healthy non-pregnant women. All the pregnant women underwent standard examination and determination of oxidative-antioxidant system indicators (malondialdehyde, total antioxidant capacity, activity of the antiperoxide enzyme catalase, and level of C-reactive protein in the serum). Results. In physiological pregnancy, free radical oxidation was ascertained to be enhanced and a new, adaptive level of the antioxidant defense system emerged. The parturient women with early preterm labor showed higher lipid peroxidation and weakened antioxidant defense mechanisms. Miscarriage in the presence of early amniorrhea without signs of a systemic inflammatory response is accompanied by enhanced lipid peroxidation processes and substantially inhibited antioxidant defense. Preterm labor in the presence of early amniorrhea and a positive blood C-reactive protein level (an acute-phase marker of inflammation) is characterized by the considerable activation of lipid peroxidation and the formation of malondialdehyde in case of a breakdown of enzymatic and overall antioxidant defense mechanisms. Conclusion. It is advisable to use the established changes in the oxidative-antioxidant system indicators in threatening and early preterm labor in further investigations. There is evidence that aseptic inflammation plays a role in the genesis of early preterm labor.