AbstractMeasurement of concentrations at which substances in combination become toxic is complex, especially if quantitative definition of additive, antagonistic or synergistric toxicity is desired. Using inhibition of oxygen utilized by aerobic micro‐organisms in their metabolism of a standard substrate, seven chemicals were tested singly and in eight combinations for their graded toxic effects. In two of the experimental investigations the combined effects of three chemicals were determined. By comparing summations of the measured effects of the chemicals tested singly with similar results from testing the toxicity of actual combinations of the chemicals in solution it was found that the actual inhibitions of the oxygen utilization were essentially the same as predicted inhibitions over a range of aqueous concentrations. Statistical analysis of all combinations tested indicated the effects to be addivtive. Neither synergism nor antagonism was shown. the pure compounds tested were: mercuric chloride, potassium cyanide, nickel chloride, potassium chromate, copper sulfate, phenol and nitrobenzene. Although the toxicity of each chemical has been quite extensively studied, results regarding combined effects are relatively few. The methods presented here are applicable to many compounds in combinatio
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