Studies on the primary drug resistance to the major anti-tuberculosis drugs in Japan were conducted by the Tuberculosis Research Committee (Ryoken) in 1966. Seventy-one institutions throughout the whole country participated. The prevalence of primary drug resistance to the major drugs among newly admitted patients with pulmonary tuberculosis to these institutions during the year 1966 was investigated. Drug sensitivity was rechecked by the reference laboratories of the Committee. Previously untreated patients comprised 35·1 % of the 10,983 newly admitted patients, and among them, 51·9% were culture-positive. The prevalence of primary drug resistance was 8·5 % according to the results of the sensitivity tests conducted at the reference laboratories. The prevalence was higher in the younger age groups, and it decreased with age. The prevalence was higher among patients with tuberculosis or previous tuberculosis deaths within the same household. The culture-negative conversion rate during treatment with streptomycin, isoniazid and PAS was compared among the sensitive and resistant groups and the group with reduced drug susceptibility. Among those with pretreatment culture (+) and (++), the rate was similar in the sensitive group, the group with reduced susceptibility and the group resistant to a single drug. The rate in the group resistant to two or three drugs was lower; and the difference was statistically significant at five months. Among those with pretreatment culture (+++) and (++++), the rate in the group resistant to a single drug was slightly lower, and the rate in the group resistant to two or three drugs was considerably lower than that of the sensitive groups and the group with reduced susceptibility. The differences were significant for the rate at six months in the former and at five and six months in the latter. Comparing the culture-negative conversion rate among 50 matched pairs composed of patients with sensitive and resistant cultures with similar background factors, the rate in the resistant group, especially those treated by the same regimen of chemotherapy throughout the six months period, was lower than that of the sensitive group; and the difference was significant for the rates from the third to the sixth month.