Consumer usage of point-of-use granular activated carbon filtration devices is increasing rapidly, yet there is little information available to the consumer concerning the efficacy of these devices. In particular, the consumer is very likely to be unaware of the fact that these devices frequently generate large numbers of bacteria that are shed in the product water. Studies of point-of-use treatment devices indicate that the bacteriological quality of product water varies both within units of the same design and among units of different design. Important factors in development of bacterial populations include the length of time that a filter cartridge is in service, water temperature, flow rate, construction materials, and the quality of the influent water. Some units contain silver as a bacteriostatic agent; its effectiveness is questionable, however, because heterotrophic bacteria concentrations associated with such units are often as high as those found for non-silver containing units. Although long-term sampling of test units indicates that a wide variety of bacteria can be isolated, including some opportunistic and frank pathogens, little or no information is available on adverse health effects to consumers from ingestion of water from point-of-use treatment devices.