There are a number of reports indicating that exposure to the toxins produced by cyanobacteria (blue-green algae) can cause acute or chronic illnesses in humans and other animals. The illnesses attributed to exposure to these toxins include gastroenteritis, respiratory and neurologic effects, skin irritation, allergic responses, and liver damage. The potential routes of exposure include skin contact, inhalation, water and food consumption, and hemodialysis. Although the evidence from reports of human health effects, in conjunction with data from laboratory animal research, suggests that cyanobacterial toxins are responsible for a range of health effects, there have been few epidemiologic studies of these associations. In addition, it is unclear which of the possible pathways of exposure are most important. For example, the extent of human exposure through drinking water is unknown but may be extensive because conventional drinking water treatment does not remove these toxins. The objective of our proposed study is to assess the public health impact of exposure to cyanobacterial toxins present in drinking water. First, we plan to assess the extent of potential human exposure by identifying public water systems where cyanobacterial blooms are likely to occur in the source water. We will collect water samples from source water, finished drinking water as it enters the distribution system, and tap water in homes (without point-of-use water filtration devices) and analyze the samples for cyanobacterial toxins. If we find measurable levels of these toxins in water samples from finished drinking water and household taps, we will develop a protocol to assess the human health effects potentially associated with this exposure.