Post-vaccination tuberculin sensitivity is being used to evaluate the immediate effects of the extensive WHO/UNICEF mass BCG vaccination programmes currently in progress. During the past five years the Tuberculosis Research Office has been studying the tuberculin sensitivity produced by BCG vaccination, and the present paper discusses some of the most important characteristics of BCG-induced allergy. The material for the paper was drawn from the results in five countries of vaccinating more than 6,000 schoolchildren and retesting them at one or more intervals after vaccination.Tuberculin sensitivity produced by BCG is not the kind of response that may logically be described as "positive" or "negative". Rather, vaccination always produces, or increases, sensitivity to tuberculin, although, with some vaccines and in some persons, the degree of sensitivity produced may be low. BCG-induced allergy can best be described by the distribution of the sizes of the tuberculin reactions and summarized by the mean and standard deviation of the distribution. The common practice of classifying post-vaccination reactions as "positive" or "negative" is biologically meaningless and may be the cause of many fallacious notions about the allergy produced by BCG.The degree of post-vaccination allergy varies with the potency of the vaccine used: a potent vaccine has been shown to produce allergy about as strong as that produced by natural infection wherever carefully controlled studies have been made. No evidence was found that allergy wanes or is lost after intradermal vaccination: the impression that it does so may often have been the consequence of the practice of revaccination and of ignoring the influence of experimental error. Unless very weak vaccines are used, there is no indication that superinfection can be identified after vaccination. The diagnostic value of the tuberculin test is thus being destroyed in many places where mass campaigns are being done, particularly in those places where a high degree of tuberculin sensitivity is being produced.