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Biomonitoring is the most powerful approach for a credible risk assessment of chemical exposure at the workplace. Because adequate certified reference materials are generally not available, the participation in inter-laboratory comparison tests is the exclusive feasibility to receive information on comparability and accuracy of biomonitoring laboratories. The presentation displays the concept of the German External Quality Assessment Scheme (GEQUAS), the requirements and limitation for the adjustment of the programme as well as an extract of the results.Parameter scheme, participation quota, target values, tolerance ranges and rates of success (certificate number/participant number) were extracted from the data and results of the last 10 runs of the proficiency test programme. Each parameter was provided in two different concentrations. Generally, the target values and the tolerance ranges were estimated by the results of several so-called reference laboratories. A successful participation was certified for a parameter if the results of the participant were found to be inside the tolerance ranges of both concentrations.162 parameters were provided in the last run (7 metals in blood, 36 inorganic parameters in urine, 12 metals in plasma, 10 mercapturic acids in urine, 11 phenolic compounds in urine, 7 amines in urine, 12 solvents in urine (headspace technique), 36 other organic parameters in urine, 11 solvents in blood (headspace technique), 15 halogenated hydrocarbons in serum, 5 N-terminal valine adducts in globin). In last runs about 200 laboratories participated, of which three-fourth were located outside Germany. The rate of success ranged for most parameters between 50% and 100% (mean 75%). Poor rate of success occurred often when conjugates had to hydrolyse before analysis.GEQUAS enables proficiency testing for the most biomonitoring parameters which are used for the exposure assessment at workplaces (metals, solvents, pesticides, new emerging chemicals). Levels and specification are carefully adjusted to the practice.