Isoniazid and iproniazid, were studied in 114 patients with pulmonary tuberculosis. Of these, 27 were treated with isoniazid alone, 53 with isoniazid and streptomycin, and 34 with iproniazid. The symptomatic improvement derived from both drugs was considerable, but it was more frequent and much more dramatic with iproniazid than with isoniazid. Essentially, it consisted of a sense of well-being, reduction of sputum, and increased appetite. In a considerable number of patients treated with isoniazid alone, initial improvement of symptoms was soon followed by a reversion to the pre-treatment symptomatic status; this did not occur with the combination of isoniazid and streptomycin. Sputum conversion occurred in 24% of the patients treated with isoniazid alone, 54% with isoniazid plus streptomycin, and 43% with iproniazid. Impressive improvement in the pulmonary pathology was limited to the fresh exudative and exudative-caseous forms of the disease. In general, improvements from the new drugs alone did not measure up to those achieved with streptomycin and PAS, and in no case were the results striking enough to be rated as superior to those from the older drugs. The effect of the combination of isoniazid and streptomycin is comparable, if not superior, to that of the streptomycin-PAS combination. The toxic side effects of isoniazid were infrequent, of minor degree, and usually temporary, while those of iproniazid were frequent and of great var., both physical and mental; in some cases the effects were severe. 17% of the patients so treated suffered from psychotic episodes. The occurrence of untoward incidents during anesthesia in iproniazid-treated patients was especially disturbing. Iproniazid should be a welcome addition in the treatment of greatly debilitated patients in whom a speedy gain of weight is of paramount importance and in whom the taking of a calculated risk is justified. It should definitely be restricted to institutional therapy.