Arylamine N-acetyltransferase (NAT) activities with 2-aminofluorene (2-AF) and p-aminobenzoic acid (PABA) as substrates were determined in Helicobacter pylori, collected from patients with peptic ulcers. The NAT activity was determined using an acetyl CoA recycling assay and high pressure liquid chromatography. Inhibition of growth studies from H. pylori demonstrated that glycyrrhizic acid elicited dose-dependent bactericidal effect in H. pylori cultures, i.e.; the greater the concentration of glycyrrhizic acid, the greater the inhibition of growth of H. pylori. Cytosols or suspensions of H. pylori with and without selected concentrations of glycyrrhizic acid co-treatment showed different percentages of 2-AF and PABA acetylation. The data indicated that there was decreased NAT activity associated with increased glycyrrhizic acid in H. pylori cytosols and intact cells. For the cytosol and intact bacteria examinations, the apparent values of Km and Vmax were decreased after co-treated with 80 M glycyrrhizic acid. This report is the first demonstration of glycyrrhizic acid inhibition of arylamine NAT activity and glycyrrhizic acid inhibition of growth in the bacterium H. pylori.