Abstract Continual-flow toxicity tests were conducted to determine the acute toxic effects of copper, zinc, and cadmium mixtures on juvenile chinook salmon Oncorhynchus tshawytscha. Median lethal concentrations during 4 days (96-hour LC50 values) were most variable for zinc (39 to 122 μg/liter), less so for cadmium (0.6 to 1.6μg/liter), and least variable for copper (26 to 34μg/liter). Sensitivities of fish to metal mixtures also were variable. No synergism occurred among various mixtures of metals; two- and three-metal combinations had additive or antagonistic toxic effects. A decrease in the copper:zinc (from 1:3 to 1:12) and copper:cadmium (from 1:0.028 to 1:0.083) ratios decreased the toxicity of mixtures. Water-quality criteria for the protection of fish against mixtures of metals probably need to be developed on a site-specific and ratio-specific basis with repetitive toxicity tests.