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The rusty-red, irregularly shaped areas known as foxing or fox spots are prevalent on rag paper used in books from the sixteenth to the nineteenth century. Foxing may vary in size from just visible spots to large areas covering most of a page, and in some cases the stain migrates through successive pages. The present study investigated the role of fungi in foxing, and the location and cause of the discoloration in these spots. Using light microscopy and staining with aqueous aniline blue, it was observed that the initial cause of the foxing was a group of separated conidia (spores) which had been deposited on the surface of the paper prior to printing and had germinated in situ during the slow drying of the paper. The colour in the foxed areas is due to an alkaline-soluble rusty-red material and an insoluble straw-coloured stain in the paper fibres. Observations of migrated stains support the theory that fox spots are part of three-dimensional stain structures which are generated in stacks of paper either during the manufacturing process or after binding. Papermakers' felts are suggested as the source of contamination, and autoxidation of lipids from the conidia is a possible cause of the discoloration of the fungal material and paper fibres. [ABSTRACT FROM AUTHOR]