We studied the sensitivity and specificity of guanine environmental tests in the evaluation of the two mite allergen levels, i.e. 2 üg/g and 10 üg/g of Der p I and Der f I, considered to be risk factors for sensitization or for the development of acute asthma. We examined 239 house-dust samples for Der p I and Der f I levels (ELISA) and guanine contents (semiquantitative guanine test and quantitative assays). All house-dust samples with class 2 or 3 guanine tests contained more than 2 üg/g of Der p I and Der f I. The probability that house-dust samples of class 2 contained more than 10 üg/g of mite allergens was 88%; it was 100% for house-dust samples of class 3. The probability that a house-dust sample of class 0 contained less than 2 üg/g of mite allergen was 87%. For each level of mite allergen, a ROC curve was constructed with the true positive rates and the false positive rates calculated by different cutoffs of guanine concentration. The cutoff point which gave the best compromise between sensitivity (76%) and specificity (89%) was 2100 üg/g for the threshold of 2 üg/g of Der p I and Der f I. For detection of a mite allergen > 10 ųg, a guanine content of 3000 üg/g gave the best compromise between sensitivity (86%) and specificity (93%). In conclusion, the guanine test represents a satisfactory environmental test, inexpensive and simple, for predicting mite allergen levels. Semiquantitative and quantitative guanine measurements are more accurate in predicting an exposure level of 10 üg/g than of the level 2 üg/g of Der p I and Der f I.