The activated sludge process is a common and useful method for treating wastewater. However, because of the necessity for aerating the sludge, the potential exists for release of a variety of viable and nonviable pollutants to the ambient atmosphere. This paper reports the results of an 8-month monitoring study in the vicinity of a large activated sludge plant in a Chicago suburb. Based on the measurements of this study, the wastewater treatment plant is a significant source of TVP (total viable particles) and total coliforms. Atmospheric TVP concentrations were elevated above background out to 1.6 km. Levels for total coliforms were reduced to background at less than 0.8 km, probably reflecting the limited viability of these bacteria in air. On the average, 95% of the viable particles discharged from the plant were greater than 2.1 micrometers, and 99% of the total coliform particles were greater than 1.1 micrometers. The health implications of these pollutants for populations living in the vicinity of the plant are not well defined. But the fact that plant emissions have the potential for relatively widespread dispersion should alert those concerned with the operation, planning, and design of activated sludge processes to incorporate the most effective control measures possible.