Abstract: Sensitive, fast-response apparatus is described for the detection of the presence of (a) aerobic or facultatively anaerobic bacteria, (b) certain body cell breakdown products in body fluids or (c) unusual contamination of the ambient by aerobic bacteria. The method employed, which enables dynamic, continuous observation of catalase-H.sub.2 O.sub.2 reaction, is based upon measuring the incremental increase of oxygen partial pressure within a hydrogen peroxide solution. The inherent catalase content in such bacteria and in most animal cells promotes the rapid decomposition of hydrogen peroxide resulting in the liberation of oxygen. This measurement of oxygen partial pressure is made directly from the hydrogen peroxide solution by means of an oxygen permeable membrane polarographic cell equipped with a cathode configuration providing improved sensitivity. The presence of small numbers of bacteria or other catalase-containing cells may be quantitatively determined by conducting the catalase-H.sub.2 O.sub.