A controlled and double-blind study to determine the efficacy of isoniazid in preventing the development of pulmonary tuberculosis among the household contacts of open cases of the disease has been carried out in a rural area of Kenya-a financially handicapped country-under realistic field conditions.A one-year course of isoniazid (5-10 mg per kg of body-weight in one daily dose) administered to the contacts of active cases gave a reduction in respect of the excretion of tubercle bacilli of the order of 90% at the end of the year. In the subsequent observation period (2-4 years) both the control and the isoniazid-treated group showed a low and similar rate of adverse findings.Previous studies by the Tuberculosis Chemotherapy and BCG Centre, Nairobi, had revealed that the risk of tuberculosis among household contacts, especially children, was high and, further, that the acceptability of the drug and the regularity of drug-taking were poor among these contacts. Even so, a very considerable prophylactic effect of isoniazid was demonstrated in the present study. It therefore seems justifiable to conclude that the treatment of household contacts with isoniazid could contribute to tuberculosis control in financially handicapped countries, particularly where a tuberculosis case-finding programme is reasonably well established.