Summary:Helicobacter pyloriis the major aetiological agent of gastroduodenitis in humans. Due to the potential importance of catalase in the growth and survival ofHelicobacter pylorion the surface of inflamed mucosae, we have characterized catalase fromH. pylorias a prelude to further studies on the function of the enzymein vivo. The catalase activity ofH. pyloriwas significantly affected by the presence of blood, serum or erythrocytes in the growth medium: the greatest activity was expressed when the bacterium was grown on medium containing serum.H. pyloricatalase is a tetramer with a subunitMrof 50000. The enzyme had a pI of 9·0–9·3, was active over a broad pH range and was stable at 56 °C. It was non-competitively inhibited by sodium azide, and had no detectable peroxidase activity. TheKmfor the purified catalase was measured as 43 · 3 mm-H2O2and theVas 60 ± 3 mmol H2O2min-1(mg protein)-1. The native catalase has absorption maxima at 280 nm and 405 nm with further minor shoulders or peaks at 510 nm, 535 nm and 625 nm, consistent with the presence of an iron-porphyrin prosthetic group.