A metabolically active biological component having mixed function oxidation activity. e.g., microsomal P-450 oxidase enzyme complex, is effective in the presence of NADPH and moisture when dispersed in a thin film of a generally neutral hydrophilic film-forming binder. e.g., gelatin, to convert a broad spectrum of carcinogenic substances, into metabolic intermediates, which can be detected, preferably by a colorimetric indicator present in the binder film or an adjacent binder film and undergoing a visible color change. The film or films can be carried, e.g., as spots by a suitable support. e.g., glass or plastic sheet forming a direct-acting self-contained badge-type indicator. The colorimetric indicator is preferably a chemically reduced dye precursor of a polyarylmethane dye or analog thereof, e.g., pararosaniline or malachite green. Such reduced dye precursors are effective colorimetric indicators for organic compounds containing an epoxide structure generally as well as epoxy-containing metabolic intermediates.
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