The quinone and cytochrome components of the respiratory chain of the microaerophilic bacterium Helicobacter pylori have been investigated. The major isoprenoid quinone was menaquinone-6, with traces of menaquinone-4; no methyl-substituted or unusual menaquinone species were found. Cell yield was highest after growth at 10% (v/v) oxygen and menaquinone levels (per dry cell mass) were maximal at 5-10% (v/v) oxygen. Helicobacter pylori cells and membranes contained b- and c-type cytochromes, but not terminal oxidases of the a- or d-types, as judged by reduced minus oxidised difference spectra. Spectra consistent with the presence of a CO-binding terminal oxidase of the cytochrome b- or o-type were obtained. The soluble fraction from disrupted cells also contained cytochrome c. There were no significant qualitative differences in the cytochrome complements of cells grown at oxygen concentrations in the range 2-15% (v/v) but putative oxidases were highest in cells grown at 5-10% (v/v) oxygen.