Seafood is one of the major food resources for human consumption in the world. The CODEX Alimentarius Commission and many jurisdictions have set maximum levels of metallic contaminants in seafood. The use of reliable methods for measurement of metallic contaminants is important in safeguarding the quality of these products and the public health. The Supplementary Comparison and parallel Pilot Study APMP.QM-S19 & P40 (Toxic Elements in Seafood) covered arsenic (0.2 mg/kg – 50 mg/kg), cadmium (0.04 mg/kg – 10 mg/kg), mercury (0.02 mg/kg – 5 mg/kg) and lead (0.04 mg/kg – 10 mg/kg) in seafood. The last CCQM or RMO key comparison / supplementary comparison in the area of metallic contaminants in seafood was organized by the Government Laboratory, Hong Kong, China (GLHK) in 2011 (APMP.QM-S5 Essential and Toxic Elements in Seafood). Hence, it was timely to organize another comparison that covers different measurands. This Supplementary Comparison (APMP.QM-S19) offers different analytical challenges (e.g. in analysis of mercury and different range of measurands) as compared to the previous comparison. Moreover, it enabled National Metrology Institutes / Designated Institutes (NMIs/DIs) that did not participate in the previous comparisons to demonstrate their measurement competencies. Evidence of successful participation in formal, relevant international comparisons is needed to document calibration and measurement capability claims (CMCs) made by national metrology institutes (NMIs) and designated institutes (DIs).