Five extracts from five plants, of which four are endemic to Turkish flora, were screened for their possible in vitro antioxidant activities by two complementary test systems, namely DPPH free radical-scavenging and beta-carotene/linoleic acid. In the first case, Pelargonium endlicherianum extract exerted greater antioxidant activity than those of other plant extracts studied with an IC50 value of 7.43 +/- 0.47 mu g/ml, followed by Hieracium cappadocicum (30.0 +/- 0.14 mu g/ml). When compared to the synthetic antioxidant BHT (18.0 +/- 0.40 mu g/ml), the methanolic extract of P. endlicherianum exhibited more than two fold greater antioxidant activity. In the beta-carotene/linoleic acid test system, the most active plant was P. endlicherianum with 72.6% +/- 2.96 inhibition rate, followed by H. cappadocicum (55.1% +/- 2.33) and Verbascum wiedemannianum (52.5% +/- 3.11). Antioxidant activities of curcumin and ascorbic acid were also determined as positive controls in parallel experiments. (c) 2005 Elsevier Ltd. All rights reserved.