The certification of a second-generation biological reference material (freeze-dried human serum) for trace element determinations is described. The material was prepared under rigorously controlled conditions to avoid extraneous additions. Analytical data were obtained by the authors as well as by numerous other intra-and extra-mural investigators, solicited on the basis of established experience in determining selected elements. For 14 trace elements (aluminium, chromium, manganese, iron, cobalt, copper, zinc, arsenic, selenmm, bromine, rubidium, molybdenum, cadmium and caesium) certified values (in ng g -i or pg g-' dry weight) are listed; for an additional element (nickel) a best estimate (m ng g -' dry weight) is added. Trace element concentrations in the material, which is available to the scientific community, closely approximate those in normal, lyophilized blood plasma or serum samples. The material thus provides the means to check the accuracy and precision of analytical procedures for quantifying low-level trace elements in the best possible conditions and to detect errors that can easily be overlooked when reference materials with higher levels of trace elements are used. In addition, and m contrast to already existing biological reference materials with high levels of trace elements, it offers the possibility of identifying unsuspected errors at the sample preparation stage.