AbstractWe have studied the mutagenicity of the water from Lake Bloomington and of the tap water that is made from the lake water. The lake, which is the source of drinking water for Bloomington, Illinois (pop. 44,000), is surrounded by land that is farmed intensively ‐ being mainly in maize and soybeans. Samples were collected monthly from May through October 1979 and concentrated 3,000× with XAD‐2 resin. Nearly all of the lake and tap water concentrates were mutagenic in Salmonella typhimurium TA98 and TA100 in the presence of S‐9 mix, and the May tap water concentrate was highly mutagenic. In addition, many of the concentrates were toxic to the bacteria in the absence of S‐9 mix. Chemical analysis of the highly mutagenic tap water concentrate from May revealed the presence of a number of organic contaminants that were absent from control concentrates prepared from deionized and distilled treated well water. In addition, unconcentrated lake and tap water were tested in a reverse‐mutation test in maize (Zea mays); no mutagenicity was detected. This study indicates that the contamination of drinking water with agricultural and/or industrial chemicals may result in a potential health hazard.