Phosphate in wastewater effluent is implicated in eutrophication of water reserves. Enhanced biological phosphate removal by activated sludge is attributed to polyphosphate accumulating bacteria, which release phosphate during anaerobiosis and reincorporate it during aerobiosis. The aim of the study was to investigate whether the process of phosphate removal by activated sludge could be probed immunochemically. Antigen preparations from the aerobic and preceding anoxic zones of a phosphate removing system contained intact and lysed bacterial cells. Neither conventional nor subtractive immunisation strategies, the latter employing cyclophosphamide to immunofocus on unique epitopes in the zones, provided antibodies capable of distinguishing between these zones. However, a putatively protein-directed monoclonal antibody could distinguish between the aerobic zones of two activated sludge systems, differing only in phosphate removal ability: immunoblot showed five discrete bands, with molecular weights appearing to be multiples of 18 kDa, unique to the system successful at phosphate removal.