Phenolic phytochemicals consumed via our diet are good sources of natural antioxidants. These phenolic metabolites have beneficial effects on human health, including inhibition of mutagenesis and carcinogenesis. In addition some phenolic phytochemicals have also been shown to have antimicrobial and antifungal activity. Oregano (Origanum vulgare L.) is an important Mediterranean herb rich in phenolic compounds with antioxidant and antimicrobial activity. Using tissue culture techniques several high phenolics and rosmarinic acid-containing oregano clonal lines have been isolated with high antioxidant activity with antimicrobial potential. In this investigation the antioxidant activity of phenolic-enriched clonal oregano extracts, and their antimicrobial activity against ulcer-associated Helicobacter pylori have been evaluated. In all cases clonal extracts were compared to commercial oregano from heterogeneous sources. Total phenolics were the highest in 60% ethanol extracts of clonal oregano. Antioxidant activity based on 1,1-diphenyl-2-picrylhydrazyl-2-radical (DPPH) scavenging activity depended on the physiochemical nature of phenolic phytochemical and percentage inhibition activity based on 3-ethyl benzthiazoline-6-sulphonic acid (ABTS(+)) assay correlated to the amount of total phenolics. Thiobarbutyric acid reactive substances (TBARS) and antioxidant protection factor (APF) method using p-carotene linoleic acid system were correlated to lipophilic phenolic phytochemicals. Antimicrobial activity against H. pylori was tested by the standard agar diffusion method. Phenolic profiles in oregano extracts were analyzed using HPLC. The differences in physico-chemical properties of phenolic acids consisting Of C-6-C-1-COOH, and C-6-C-3-COOH structures were hypothesized to play a role in growth inhibition of H. pylori. (C) 2004 Elsevier Ltd. All rights reserved.