The aim of the study was to assess ambient ozone phytotoxicity in Kyiv (Ukraine) using bioindicator clover plants (Trifolium subterraneum cv. Geraldton) and to test some natural and synthetic antioxidants as ozone protectants. The results obtained showed that ambient ozone concentrations were high enough to cause visible leaf injury in clover. All used substances showed partial ozone protective effect on clover. Water extracts from the leaves of plants, known to contain flavonoids-antioxidants showed weaker ozone protective effect and were less stable in the field conditions than synthetic antioxidants. Among the studied extracts, those from Ocimum basilicum and Tagetes patula were more effective as ozone protectants than the one from Salvia sclarea. (C) 2007 Elsevier B.V. All rights reserved.