A screening assay for recognizing the presence of dioxins (and other related toxins) in a sample is disclosed. In one aspect of the invention, Ah receptor from mice and a radioactively labelled halogenated dioxin are used in a competitive binding assay to test for the presence of toxins. The label is 125 I substituted directly on the main dioxin structure. The relative binding of the toxin in the samples (in competition with labelled dioxin) for Ah receptor can be compared against standard curves. A kit is provided for running such an assay and a preferred 125 I ligand is provided.

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