The treatment of the complex of diseases known as leishmaniasis, caused by infection with protozoal parasites, is often unsatisfactory. The response to chemotherapy depends not only on the species of infecting Leishmania but also on the immune reactions of the host. This creates difficulties in evaluating any new drug in clinical trials. In a Brazilian patient with diffuse cutaneous leishmaniasis, which classical antileishmanial drugs had failed to cure, antituberculous therapy of an intercurrent mycobacterial infection produced a striking remission of the leishmanial skin lesions. Subsequent studies in mice infected with the strain of L. mexicana amazonensis isolated from this patient showed that para-aminosalicylic acid was inactive and both rifampicin and isoniazid had only a moderate beneficial action. However, a pronounced degree of potentiation was observed when rifampicin and isoniazid were administered in combination. Clinical trials of this combination are recommended in further patients, as well as experimental studies on other antimycobacterial agents, alone and in various combinations.