In its seventh report, published in 1960, the WHO Expert Committee on Tuberculosis "noted the need for international standards for the definition and determination of drug resistance which will permit comparisons to be made from one area to another, and recommended that the World Health Organization take appropriate steps to establish such standards".(10) Acting on this recommendation, WHO took the first step towards standardization by convening in Geneva, in December 1961, an informal international meeting of specialists in the bacteriology of tuberculosis. At this meeting an attempt was made to formulate prerequisites for reliable sensitivity tests and to specify the technical procedures for them.The first part of the present paper is a joint contribution by the participants in the meeting, summarizing the general conclusions reached and recommendations made with regard to tests of sensitivity to the three main antituberculosis drugs-isoniazid, streptomycin and p-aminosalicylic acid. The other three parts describe, in turn, three different tests for determining drug sensitivity-the absolute-concentration method, the resistance-ratio method and the proportion method-that are generally considered to give reasonably accurate results.