Effect of specific scavengers of reactive oxygen species on free-radical oxidation is studied in central and peripheral zones of the placenta from preterm delivering women (32-36 weeks) using the chemiluminescence method. A lower contribution of hypochlorite into free-radical processes and a reduced content of nonprotein SH-groups in the placenta are observed, superoxide dismutase, catalase, and total antioxidant activity being unchanged. It can be assumed that the reduced contribution of hypochlorite into free-radical processes is partially responsible for impaired antimicrobial barrier between mother and fetus in preterm labor.