Describes interlaboratory studies and quality assurance/control procedures related to the analysis of environmental samples for the Northern River Basins Study. The interlaboratory studies involved sending samples to be analyzed for dioxin and furan congeners, polyaromatic hydrocarbons, polychlorinated biphenyls, resin acids, metals, and chlorinated phenols. Laboratories were evaluated on closeness to mean and expected concentrations, surrogate recoveries, detection limits, sample handling, dedicated facilities, subcontracted work, high- versus low-resolution mass spectrography results, additional quality assurance/control effort, non-detect data that was detected by other laboratories, flexibility, and treatment of outliers. Appendices include analytical and sampling protocols, plus terms of reference for analytical services.