The number of persons in various risk categories who must be given isoniazid chemoprophylaxis to prevent one case of active tuberculosis has been estimated in order to help decide on indications for chemoprophylaxis. Assuming the protective effect of chemoprophylaxis is limited to 10 years, this number is 14.2 for persons with inactive cases of tuberculosis, 37.5 for primary tuberculosis in children, 81.3 for household contacts of active cases, 110.3 for tuberculin-positive adolescents with normal chest roentgenograms, and 163.4 for tuberculin-positive adults with normal chest roentgenograms. The cost to prevent one case in each of these categories is $824, $1,612, $813, $4,743, and $7,026, respectively. When weighing this information plus the knowledge of a significant but low frequency of isoniazid toxicity against the cost of treating an active case of tuberculosis and the complications of such treatment, a strong recommendation can be made to give preventive treatment to all cases of primary tuberculosis in children and all inactive cases of tuberculosis. By contrast, the justification for giving isoniazid to all tuberculin-positive adults with normal chest roentgenograms seems weak. In intermediate situations, such as all household contacts to persons with active disease, all tuberculin-positive adolescents with normal chest films, and selected tuberculinpositive adults with normal chest films who have additional conditions that increase the risk of developing active tuberculosis, chemoprophylaxis is probably justified unless isoniazid proves to be more toxic than is now established.