AbstractTwo 30 dm3coastal seawater samples were collected in September 1983. One was acidified, homogenized and divided among 50 500‐cm3glass ampoules. The second was spiked with 612ng of mercury added as methylmercury chloride, acidified, homogenized and divided among 50 500‐cm3glass ampoules. All these operations were performed under a mercury‐free atmosphere. A single sample from each set was sent to each of 37 laboratories; 21 of them submitted their results of a ‘total’ mercury concentation determination. The consensus values and their confidence limits are 6.0 ± 1.6 and 24.2 ± 2.8 ng dm−3for the natural and the spiked samples respectively. Eleven laboratories reported reliable quantitative results (within ± 2SD) for the natural (unspiked) sample, and 12 for the spiked sample. In addition, ten laboratories give sets of results within ± 2SDof the consensus values. Storage for up to four months of methylmercury spiked samples with 1.4% nitric acid (0.3 mol dm−3) at room temperature is, alone, an unsuitable treatment for releasing mercury from its mon