Results from an interlaboratory calibration exercise on the reproducibility in the determination of cadmium, chromium, copper, iron, manganese, nickel, lead, and zinc in marine plant material are presented. The fourteen participating laboratories each analysed the heavy metal content in two samples of dried eelgrass material (spiked and unspiked), a predigested sample, and a mixed standard solution.Significant variations in the reported heavy metal concentrations were observed. The variability between laboratories was greatest for cadmium, chromium, nickel, and lead, whereas the variability for iron, manganese, and zinc was less. The reproducibility in reported heavy metal concentrations was generally best in the mixed standard solution and poorest in the dried eelgrass samples. Statistical analyses showed that some laboratories generally reported values closer to the overall means than did others.The sources of variation are discussed, and recommendations to improve the quality of heavy metal analyses are given.