Assessing and achieving interlaboratory comparability is a major quality assurance challenge for large interdisciplinary, multi-institutional, environmental research projects. Eight laboratories routinely measure chemical constituents of various field and laboratory derived solution samples as part of the US EPA's Watershed Manipulation Project. A quarterly interlaboratory comparison study has been designed by EPA quality assurance personnel in Corvallis, OR (USA) to assess: (1) intralaboratory precision and accuracy, (2) interlaboratory precision, and (3) trends through time. Synthetic solution samples are prepared quarterly and sent via overnight express to participating laboratories. For each analyte, laboratories are requested to perform analyses in triplicate with each replicate measured under different calibrations; precision calculated from resulting data therefore includes a component of among-batch variability. Relative bias is calculated as the percent difference between a laboratory's mean and the median of the laboratories (the supplying laboratory's value being treated equally to the other laboratories). Overall precision is expressed as the coefficient of variation using laboratory means. An ANOVA approach is also used to calculate significant differences among laboratories. Data interpretation and results from four quarterly evaluations will be presented.