The authors describe the methodology of allergen challenge tests, with special attention to some particular aspects, such as the allergenic extracts and their biological activity, and the methods used in aerosol administration and in measurements of bronchial obstruction. They present the results of their personal experience, concerning interindividual variations in a sensitized population and the reproducibility of the immediate reaction revealed by dose-response curves. Comparisons of allergen tests with exposure to allergens under physiological conditions are discussed. In occupational asthma the exposure tests sometimes constitute the only diagnostic method, owing to ignorance of the responsible mechanisms and to the multiplicity of incriminated substances. However, these tests are not without danger and must be performed by well-trained medical and paramedical teams.