The present state of knowledge on the chemotherapy for plant virus diseases has recently been considerably broadened. The discovery of purines and purine analogs has made it possible, with greater success, to provide a reliable information on this subject. Furthermore, R.A. Gray has reported a new antibiotic, noformicin, which inhibits both the production of local lesions and systemic infections caused by southern bean mosaic virus and by tobacco mosaic virus in intact plants. In the following are presented the results of the studies, although the preliminary nature, obtained in an effort to extend a fundamental investigation on this approach, especially on the effect of the new antibiotics upon the tobacco mosaic virus (TMV) multiplication in tobacco leaf tissues. The mature leaves which had been inoculated one day ago by rubbing with a gauze pad soaked in TMV-infected tobacco juice, were removed from tobacco plants (Nicotiana tabacum, Turkish) and split along the midrib. One-half of each detached leaf was floated on the solution containing the antibiotics in Petri dishes under fluorescent lamp in an incubator at 20-25°C. The opposite leaf-halves which had been inoculated with the same virus were kept on water and served as controls. After 3-4 days incubation, the leaf tissues were removed and ground in a mortar for virus analysis. The procedure shown in figure 1 was undertaken for precipitating virus protein, the relative concentration of which in each samples was determined by a colorimetric procedure using the Folin reagent. Readings were obtained with an electrophotometer. Phytotoxicity of the antibiotics: Injury symptoms were noted for each antibiotics over a wide range of concentrations. Moreover, the rate of respiration of leaf disk floated on the antibiotic solution was measured by using Warburg's manometer and was compared with the control disk placed on water. Antibiotic translocation: Translocation in the detached tobacco leaves of the antibiotic solution was ascertained. Leaf tisssues floated on the solution were crushed with mortar and pestle to secure an extract for antibiotic assay. These extracts were assayed by the cup-method using the test organisms for each antibiotics. Inhibition zones were measured after 18 hours incubation at 37°C. The antibiotics used in these studies were supplied from the Research Laboratory, Takeda Pharmaceutical Industries, Ltd. and most of them were the crude substances. Some of the results obtained are summarized in tables 1 and 2. As seen in the tables, 24 samples out of 62 samples tested were found to exhibit 20 per cent or more virus-inhibition. Most effective inhibition to virus multiplication was 58 per cent shown by the sample No. 62, whereas 8-azaguanine (10-3M) and noformicin (200 ppm, crude), the latter was kindly supplied by Dr. R.A.Gray, showed 32 and 39 per cent virus-inhibition, respectively, under our experimental conditions. The successive experiments, applying the antibiotic solution by leaf-spraying method and so on, are now under progress.