Since 1979, the Centre de Toxicologie du Québec has operated an interlaboratory comparison programme for several toxic substances in blood and urine. We initiated this program due to the unavailability of reliable and representative reference materials for toxic elements in blood and urine. The main objective is to enable participants to improve or maintain the accuracy of their analytical work by periodically comparing their results. There are presently 76 laboratories registered in the programme for one or more substances. The majority are located in North America with several others in Europe and South America. Available substances include lead and cadmium in blood, aluminum in serum, and mercury, arsenic, cadmium, fluoride and chromium in urine. Samples are prepared by pooling specimens obtained from exposed workers. We have examined the performance of participants over the course of the years using as criteria the deviation from the target value, the scatter of results and the proficiency within preset limits of toxicological significance. Whenever possible the influence of analytical methods was evaluated. The feedback provided by participation in the programme appears to be a significant factor in the maintenance or improvement of analytical proficiency. The vast majority of participants obtain results of adequate reliability for use in the monitoring of exposed workers.