In order to bring light to the mechanism of weakening of resistance to INH, the author cultured several generations of a perfect INH-resistant single colony of bacilli, conducted experiments on several generations of mixed population, further cultured generations of bacilli in a poorly fed culture medium and studied the change of population and thoroughly examined the factor in culture medium which has effects on the weakening of resistance. The results of the experiments observed were as follows: 1. No weakening of resistance could be observed in INH 10γ resistant strains obtained by one step, step by step cultured or by direct separation from patient if strains have been cultured in 1% KH(2)PO(4) medium. 2. No weakening of resistance could be observed by testing generations of strains, some of which were obtained by step by step culture and some by means of separation from patient, in a poorly fed culture medium. 3. According to the method of mixed population, the ratio of INH resistant bacilli contained therein decreased as generation of bacilli cultured in a 1% KH(2)PO(4) medium descends. 4. The ratio of INH resistant bacilli in mixed population cultured in a poorly fed medium rapidly decreased as generation descends compared to corresponding generation of bacilli cultured in 1% KH(2)PO(4) medium. However, the difference in viability between INH resistant bacilli and INH receptive bacilli cultured in a poorly fed medium could not be observed clearly. Neither could any mutagenic effect on the INH resistance of bacilli on a poorly fed culture medium be found. 5. Upon finding the rapid weakening of the resistance of INH resistant bacilli cultured in a poorly medium, the author prepared a glycerine lacking medium and also a sodium glutamine lacking medium and conducted experiments on generations of bacilli cultured in such mediums to determine the factors in the medium which cause the weakening. The author found that in the former case (glycerine lacking medium) weakening is delayed whereas in the latter case weakening process is observed promptly. It is presumed that INH resistant bacilli requires glutamic acid far more than INH receptive bacilli and that for that reason the growth of bacilli in a sodium glutamine lacking medium is delayed.