1. Lethal limits of concentration are determined for lead, zinc and copper for the three-spined stickleback (Gasterosteus aculeatus L.).

2. The addition of calcium salts to solutions of lead nitrate or zinc sulphate reduces the toxicity of these salts to Gasterosteus. 50 mg. per l. of calcium (as nitrate or chloride) is sufficient to annul the toxicity of a 1 x 10-6 g. per c.c. solution of lead or a 2 x 10-6 g. per c.c. solution of zinc.

3. The reactions of the fish in the solutions with and without calcium are compared, and the respiratory symptoms are described with the aid of graphs illustrating the variation in respiration rate during the survival time.

4. It is shown that a running supply of "hard" tap water containing approximately 50 mg. per l. of calcium as calcium bicarbonate is harmless to the minnow, Phoxinus phoxinus (L.) and to the stickleback, when there is added to it the maximum amount of lead that it can hold in solution (7 x 10-7g. per c.c.). This concentration of lead in soft water is fatal to Gasterosteus in 38½ hr.

5. The same amount of calcium renders a 10 x 10-6g. per c.c. solution of lead harmless to the goldfish, Carassius auratus (L.).

6. It is concluded that, in the presence of sufficient calcium, the interaction between the lead, or zinc, and the mucus secreted by the fish does not take place. This conclusion was endorsed by experiments in vitro on the slime secreted by the eel, Anguilla anguilla (L.).

7. The application of these results to the pollution of natural waters by effluents from lead and zinc workings is briefly discussed.

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