Many antibiotics of bacterial origin had been found and most of them were polypeptide such as bacitracin, gramicidin, polymixin, etc. As reported in the previous paper¹⁾, a new antibiotic was obtained from bacterium as white needle crystals.

The source of the antibiotic, strain No. 203, was isolated from the soil in a vineyard at Aizusakashita-machi, Fukushima-ken and it was characterized to be a species of Bacillus megatherium.

This antibiotic, effective against yeasts and bacteria, was confirmed to be a new antibiotic on the basis of its physical, chemical and biological properties, and named bacimethrin.

Mode of action of angustmycins and mikamycins has been studied in our laboratory. As reported in the previous papers^1～5), angustmycins interfered with nucleic acid synthesis, particularly guanosine synthesis. The primary site of action was suggested to be the inhibition of transformation process of xanthosine monophophate to guanosme monophosphate. Mikamycin, especially mikamycin A, inhibited protein synthesis on the ribosome. A marked synergistic activity of mikamycins A and B was demonstrated in the inhibition of protein synthesis as well as in growth inhibition of sensitive organisms.

Gourevitch, et al.⁶⁾ reported that the major biochemical lesion caused by kanamycin was associated with the oxidative respiratory pathway, because it was more active under aerobic than anaerobic conditions and acids of tricarboxylic acid cycle were antagonistic to the growth-inhibitory action of kanamycin. They also observed an increased osmotic fragility of kanamycin-treated organisms. Aoki, et al.^7,8) described that kanamycin inhibited oxidation of benzoate, niacin, malonate, malate, fumarate, succinate and putrescine by kanamycin-sensitive avian tubercle bacilli in higher concentrations than the minimum growth-inhibitory concentration. Kanamycin-resistant organisms were more resistant to the inhibitory effects on the oxidation. Tsukamura, et al.¹⁵⁾ observed an inhibition of ³²P and ³⁵S incorporation into a mycobacterium by kanamycin.

In the series of our studies on the mechanism of action of miscellaneous antibiotics, an inhibitory effect of kanamycin on protein synthesis was observed. The effect was compared with that of dihydrostreptomycin. In this publication, the experimental results are presented and also the relation to the growth-inhibitory effect is discussed.

A family of new antibiotics, named rifamycins have been recently discovered by Sensi, et al.^1～4) Rifamycin SV, a derivative of rifamycin B, is highly active against Gram-positive bacteria and tubercle bacilli in in vitro experiments⁵⁾. Its toxicity in animals is extremely low⁶⁾.

Timbal and Brega⁷⁾ stated that rifamycin SV delayed the mortality of mice infected with tubercle bacilli. Monaldi, Curci and Nitti⁸⁾ made basic studies on the antituberculous action of this drug and reported that this drug was effective against experimental tuberculosis in guinea pigs.

The purpose of the present study is to deal with the antituberculous action of this drug and to present some findings on the nature of action.