The U.S. Geological Survey maintains a quality-assurance program based on the analysis of reference sam¬ ples for its National Water-Quality Laboratory located in Denver, Colorado. Reference samples containing selected inorganic constituents are prepared at the Survey's Water Quality Services Unit in Ocala, Florida, disguised as rou¬ tine samples, and sent daily or weekly, as appropriate, to the laboratory through other Survey offices. Nutrient sam¬ ples and precipitation samples also were submitted as samples of unknown concentration. The results are stored permanently in the National Water Data Storage and Retrieval System (WATSTORE), the Survey's data base for all water data. These data are analyzed statistically for precision and bias. The results of these statistical analyses are discussed for data collected during water-year 1987. An overall evaluation of the major and trace constituent data for water-year 1987 indicated a lack of precision in the National Water-Quality Laboratory for the determination of 6 out of 58 constituents: chloride; chromium; iron, total recoverable; zinc, dissolved (atomic absorption spectroscopy); zinc, (inductively coupled plasma emission spectroscopy); and zinc, total recoverable. There were fewer constituents having positive or negative bias during water-year 1987 than during water-year 1986. A lack of precision was indicated in the determination of three of the six nutrient constituents: nitrate + nitrite nitrogen as N, orthophosphate as R and phosphorus as R A biased condition was indicated in the determination of ammonia + organic nitrogen as N. There was acceptable precision in the determination of all 10 precipitation-level constituents. One precipita¬ tion-level constituent, sodium, indicated a biased condition.