This report embraces the clinical experiences with 76 pulmonary tuberculous patients treated with isoniazide and/or iproniazide. The cases were divided into five groups: Group I (parent group) comprised 59 patients, of whom 34 at one time had streptomycin alone or in combination with PAS and the remaining 25 never had been treated with these drugs. They received isoniazide for a 90 day period on the following dosage schedule: 1.0 mg./Kilo body of weight for the first 30 days, 2.0 mg./Kilo for the second 30 days and 4.0 mg./Kilo for the last 30 days. The drug was given daily by mouth in three divided boses. After 90 days, 39 cases were available and were regrouped as follows: (a) Group II (22 cases) who showed roentgenological improvement earlier were continued for another 90 days on isoniazide in the dosage of 4.0 mg./Kilo; (b) Group III (six cases) had never received streptomycin and had shown no regressive roentgen changes with isoniazide alone for 90 days. They were subsequently given streptomycin gram 1.0 every third day plus isoniazide daily (4.0 mg./Kilo) over a span of 90 days; (c) Group IV (11 cases) showed no improvement on the chest x-ray film with isoniazide alone but previously had received streptomycin. These cases were placed on iproniazide alone in the dosage of 1, 2, 4 mg./Kilo body of weight for a 90 day period similar to that employed for Group I. Group V, an independent category of 17 cases, did not have isoniazide at any time. Instead, these patients received iproniazide for 90 days, again in the same manner as the preceding group. Iproniazide was found to be superior to isoniazide in producing a more prompt reduction in febrile toxicity and in accelerating gain in weight. There was no significant difference in the regressive x-ray film changes noted with 90 days of therapy in Group I (52 per cent) as compared to Group V (59 per cent). Of the 39 cases followed in Groups II, III, IV, a total of 19, approximately 50 per cent, showed further improvement roentgenologically. The course of 180 days of isoniazide alone appeared superior in this respect than did the other regimens. However, in the streptomycin plus isoniazide treated group, there was not a single case that showed progression of the lesion. The most favorable therapeutic effect was observed in the resolution (of varying degree) of the exudative component of the parenchymal process. While reduction in size of cavitation was seen in a number of instances, non-visualization of cavity on laminography was found in only one of the 76 cases treated by the various regimens. After 90 days of therapy, the incidence of sputum conversion was three times higher with isoniazide (36 per cent) than with iproniazide (12 per cent). After 180 days of treatment in Groups II, III, IV, the conversion rate dropped considerably and averaged about 16 per cent. Beneficial subjective and objective effects were seen with as low as 1.0 mg./Kilo body of weight daily of either hydrazide preparation. The dose of isoniazide now being used by us is 4.0 mg./Kilo daily. Clinical manifestations of toxicity in the isoniazide treated cases occurred in 46 per cent and in the iproniazide group in 61 per cent. The mostcommon symptom encountered was vertigo. The symptoms with isoniazide were mild and transient while those with iproniazide tended to be more severe and progressive. For this reason, the latter drug is not advocated at present in the therapy of the average case of pulmonary tuberculosis. Laboratory findings indicative of possible toxicity have been discussed. The number of bacterial sensitivity examinations made in this study were inadequate to form any conclusions. Other reports indicate a relatively early appearance of resistant strains of tubercle bacilli when patients are treated with isoniazide alone—and the occurrence of cross resistance between this drug and iproniazide. Isoniazide is a useful drug in pulmonary tuberculosis but is limited apparently by this factor of bacterial resistance. Further studies are necessary to determine its efficacy in combination with streptomycin or other drugs and its comparative value with the standard combination of streptomycin and PAS.