Deterioration of paper accompanied by staining is called foxing. Reports on research carried out into possible causes of foxing stains on paper. Covers the following topics: nonliving factors which bring about the discolouration of paper; fungi on paper; the iron content of paper; iron and the growth of fungi on paper; discolouration by pigment production; sizing and filler with reference to growth of fungi on paper; pH of paper and fungus growth; moisture and growth of fungi; air-conditioning in libraries; histological evidence of the presence of fungus in foxed paper; digestion of cellulose by fungi; the nature of certain products of deterioration of paper; fungicides with paper. A biological origin for foxing is postulated with the fungal growth and development of colour favoured by the presence of iron and certain sizings and fillers. Bibliography. -- ICCROM