Since guanine is not only an essential constituent of vital nucleic acids, but also the main end product of nitrogenous waste excretion in arachnids, it is a potential candidate for a hygienic index for mite activity in house dust. The public health significance of these mites is based on their production of allergen in the home environment. In 101 mattress-dust samples, total number of mites and guanine content were positively correlated. The hygienic standard of 10 mites per 0.1 g dust (as derived from Danish investigations) was met in all samples with less than 0.06% guanine in the dry dust. Where the guanine content was more than or equal to 0.25% in the dry dust, mite numbers were higher than 10 mites per 0.1 g dust in 43 of the 44 samples. The three different groups (negligible mite content, possibly allergologicaUy relevant mite concentration and allergologically unhygienic mite situation) could be easily discriminated on the color scale. The guanine-AZO-dye reaction in vacuum cleaner dust is a strong and promising candidate for an allergologically relevant hygienic index in the home.