A bioassay, using the mold fungus Aspergillus niger, gave results that were correlated with amounts of pentachlorophenol and tributyltin oxide in pine sapwood treated with solutions ranging in strength from 0.016 or below to 5%. Limited bioassay estimates of penta in commercially treated millwork corresponded to estimates by lime ignition. The assay fungus exhibited somewhat greater tolerance of the preservatives than did two prominent decay and stain fungi in comparably conducted bioassays. The procedure is simple, requires few aseptic precautions, and can be carried out in 7 to 10 days. The amount of preservative in a small specimen of wood to be assayed is indicated by the size of the area around the specimen resting on nutrient agar seeded with Aspergillus niger spores over which the fungus is unable to develop its typical black spores.

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