Using the Western blot immunochemical analysis method, the heat shock protein, GroEL, was found to be either induced or repressed in activated sludge microorganisms exposed to a range of xenobiotics. At the EC25 concentration, pentachlorophenol (PCP), cadmium, nickel, 2,4-dichloroaniline, benzoquinone, 2,4-dinitrophenol, and 1,1,1-trichloroethane all rapidly induced measurable GroEL expression, even though the time-dependent response for each of these compounds was somewhat varied. Toluene and hydroquinone resulted in repression of GroEL expression to levels below that measured in the control mixed liquor. For PCP concentrations at or exceeding the EC25, there was a significant and consistent increase in effluent volatile suspended solids from activated sludge sequencing batch reactors relative to unstressed controls. These preliminary results indicate that stress proteins may serve as sensitive and rapid indicators of toxicity which can adversely impact treatment process performance in activated sludge systems.