With the purpose to determine the fluctuation of pH. of the stool after drinking "Yakult", cow's milk fermented by some sort of "Family Lactbacillaceae", the authors made normal adults and patients with lung tuberculosis drinking one bottle (60 cc) a day of Yakult successively, and estimated the pH. of their stool colorimetrically and obtained following results. 1. In the case of normal adults taking a bottle (60 cc) of Yakult a day successively, no marked fluctuation can be recognized. 2. In the case of patients with lung tuberculosis receiving drug treatment (chemotherapy) pH. range was 6.0~7.0 namely marked difference from normal adults can be recognized. 3. In the case of patients with lung tuberculosis receiving chemotherapy, after drinking Yakult, pH. of their stool become equal with that of normal adults'. By drinking Yakult marked effect was recognized.