Change in light output of Photobacterium phosphoreum was used as a screening test for detecting the level of toxicity in clarified sewage, SBR-treated effluent, and mixed liquors. Besides using the 2% and 90% Microtox screening test protocols, a 45% screening test procedure was developed. Previously reported studies on SBR removal of BOD, COD, TSS and selected microorganisms have shown that effective and stable removal of these would generally require two or more hours of REACT. This would also result in good effluent quality in terms of toxicity. The removal of toxicants appeared to be due to their adsorption onto the biomass during FILL followed by their conversion to less- or non-toxic products and/or the assimilation and accumulation of such toxicants in the sludge. Some desorption of toxicants was observed during SETTLE. Efficient removal of toxicity, in excess of 90%, may be achieved with a REA CT of 1.5 h or more.