The 96-hr LC50 of copper to Atlantic salmon parr in water taken from the Exploits River, Newfoundland, is about 125 μg/liter at a total hardness of 8–10 mg/liter CaCO3 and a total alkalinity of 4 mg/liter. However, the addition of spent sulphite liquor from a pulp mill allowed fish to survive more than 180 μg/liter Cu. Considering the variety of organic compounds which have now been shown to chelate dissolved copper, it is likely that a simple model for the prediction of copper toxicity in waters receiving mine effluent will prove impossible.