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We tested whether multicomponent mixtures of xenoestrogens would produce significant effects when each component was combined at concentrations below its individual NOEC or EC₀₁ level. The estrogenic effects of eight chemicals of environmental relevance, including hydroxylated PCBs, benzophenones, parabenes, bisphenol A, and genistein, were recorded using a recombinant yeast estrogen screen (YES). To ensure that no chemical contributed disproportionately to the overall combination effect, a mixture was prepared at a mixture ratio proportional to the potency of each individual component. The performance of four approaches for the calculation of additive combination effects (concentration addition, toxicity equivalency factors, effect summation, and independent action) was compared. Experimental testing of the predictions revealed that concentration addition and its application, the toxicity …