In a paper arising out of an informal international consultation of specialists in the bacteriology of tuberculosis held in 1961, an attempt was made to formulate criteria, and specify technical procedures, for reliable tests of sensitivity (the absolute-concentration method, the resistance-ratio method and the proportion method) to the 3 main antituberculosis drugs (isoniazid, streptomycin and p-aminosalicylic acid). Seven years later, a further consultation was held to review the latest developments in the field and to suggest how sensitivity tests might be put to practical use in tuberculosis control programmes. The participants reached agreement on how to define drug sensitivity and resistance, and stressed the importance of using a discrimination approach to the calibration of sensitivity tests. Their views are contained in the present paper, which also includes descriptions of the sensitivity tests used by the Medical Research Council of Great Britain for first- and second-line drugs (minimal inhibitory concentration and resistance-ratio methods), the two main variants of the proportion method developed by the Institut Pasteur, Paris, and a method for calibrating sensitivity tests.