A proficiency test on the quantification of Al, Cu, Mn, Se and Zn in serum was carried out to verify the performance of about 30 regional laboratories of the network of Italian laboratories. The exercise consisted of four runs in which the laboratories were free in choosing analytical methods to determine trace elements in freeze-dried animal serum. Laboratories performances were evaluated by the study of statistical functions as Coefficients of Variation (CV), Youden plot and Z-score value. As for Al, the results were generally characterized by good accuracy and precision, in spite of the low levels of the element (5-7 microg l(-1)). Copper determination had some problems only at low concentration (about 160 microg l(-1)--first run), in which an elevated number of anomalous data were found. Better data were achieved for Zn, for which anomalous values were mainly stragglers than outliers. Due to the low number of data for Mn (concentrations from 0.6 to 60 microg l(-1)) and Se (concentration from 45 to 106 microg l(-1)), a restricted statistical treatment was applied; for these elements high CV values were found (range from 30 to 80%). The results of this trial confirmed that participation in a proficiency test represents a benefit for all analytical laboratories. In fact, with few exceptions, most of the participants improved their performances in terms of Z-score values.