Stress resulting from a variety of chemical and physical environments has been recognized in indicator bacteria. A review by Busta (1976) summarizes the extensive work that has been carried out to describe indicator microorganisms sublethally impaired due to a variety of causes associated with foods. Workers in the area of water microbiology are also gaining an appreciation of the importance of these stressed cells in the assessment of water quality using bacterial indicators. Chemical agents, including chlorine, that are employed in water disinfection processes are important causes of bacterial stress injury. As a result, a significant portion of the total population of indicator bacteria in water might not be enumerated (using the selective procedures that are currently employed) and inaccurate water quality determinations could result. Alternative water disinfection agents that are being suggested, such as ozone, chlorine dioxide, and ultraviolet irradiation, will also probably lead to the same result. In addition, heat from thermal pollution and interactions with other microorganisms or chemicals (including disinfectants and metals) also exert stress that could further debilitate indicator bacteria in various waters and effluents. A need for improved enumeration procedures has accompanied the recognition of injured indicator bacteria in chlorinated waters and wastewaters. This movement has also stimulated interest in the underlying mechanism of cellular damage that is responsible for the submaximal recovery of coliforms from disinfected waters. Various groups have reported that a number of biochemical, genetic, and physiological processes are impaired by chlorine exposure under differing conditions. Evidence from our laboratory and elsewhere implicates functions associated with the cell envelope, i.e., the uptake of extracellular organic substrates, as the primary cellular target of chlorine under conditions that are similar to those in the field. Additional data from our group indicate that sublethal damage from chlorine can be reversed under suitable nonselective conditions. Recent efforts have led to the development of new methods to enumerate injured fecal streptococcus, total and fecal coliform bacteria from chlorinated waters and wastewater. These procedures each yield data that are comparable with that obtained using the more cumbersome MPN method. As a result, the best characteristics of both methods may now be found in three relatively simple MF procedures. Some of these advances have been described in a new section (#921) of the fifteenth edition of "Standard Methods for the Examination of Water and Wastewater" entitled "Stressed Organisms" (APHA, 1981). However, it is anticipated that new and better water quality assessment methodologies will emerge from the growing literature concerning the physiological and biochemical behavior of indicator microorganisms in water and wastewater.(ABSTRACT TRUNCATED AT 400 WORDS)