The report presents water-quality and well-site information for public water-supply wells in Illinois. These wells were sampled during the period January 1985 through June 1987 as part of an ongoing cooperative ground-water observation network in the State. Water-quality data are tabulated for physical parameters, nutrients, common constituents, metals, phenols, cyanide, and volative organic compounds. A subnetwork of wells also have data on several pesticides and herbicides. Some well-site information is also presented in this report.