Abstract: An apparatus is provided for rapidly determining the biochemical oxidation demand (BOD) of a liquid at an on-site location. The apparatus includes a cell body member having a vertically extensive plunger chamber formed therein and a sample holding chamber communicating therewith, a dissolved oxygen sensor for generating a signal in proportion to the amount of dissolved oxygen content in a liquid sample and a culture medium disposed in the sample holding chamber, and a tubular plunger member extensive into the plunger chamber for supporting the dissolved oxygen sensor such that the dissolved oxygen sensor extends into the sample chamber and into contact with the liquid sample and the culture medium.