Kastela Bay, located on the central part of east Adriatic coast, is heavily contaminated by mercury. The main source of contamination was a chlor-alkali plant, which was operating for 40 years since 1949. Previous studies showed that mercury concentrated in the sediment close to the plant is being dispersed throughout the bay by remobilization processes. Subsequent remobilization of mercury buried in the sediment may be harmful to various marine organisms. In order to assess the availability of remobilised mercury to marine organisms and to assess the health risk of consumption of shellfish species from the bay, a monitoring programme was initiated in September 1997. One thousand six hundred specimens of mussels (Mytilus galloprotincialis) of the same size and age, collected in a very clean area, were transferred to four stations in the bay 1.5 m above the sea bottom. Every month during the summer period and every second month in the winter period, 50 mussels were taken from each station for the analysis of mercury and monomethylmercury (MeHg) content. The results obtained during 1 year of biomonitoring suggest that the equilibrium concentration in transplanted mussels was established in a relative short period of time. The digestive gland is preferential organ for the accumulation of total mercury. Seasonal variation of mercury content both in the whole soft tissue, and organs (gills and digestive gland) was observed at all stations. However, methyl mercury is more concentrated in soft tissues than in digestive gland and gills, despite the fact that it is absorbed through these organs. The mercury levels in tissues were below accepted limits for human consumption. Obtained results showed that the implantation of mussels in an area containing sediment contaminated by mercury may be a good monitoring tool for the assessment of the availability of remobilised mercury to marine organisms.