Available evidence indicates that mercury associated with drilling-fluid barite is mainly present in metal sulfides with very low solubility, is not detectably leached into seawater, and has limited availability to marine organisms. Direct exposure of fish, clams, sand worms, and shrimp to mixtures of drilling-fluid barite and sediment showed that no statistically significant uptake of mercury occurred in fish. Only small amounts of mercury were taken up by other organisms, possibly in the form of barite particles in their digestive tracts rather than as mercury absorbed into tissue.