A strain, which had been selected by the systematic screening of about 2,000 soil actinomycetes, was found to produce two antifungal substances. These substances were obtained in crystalline forms. One of them, the antifungal substance, No. 720-A, was studied in detail and confirmed to be identical with antimycin A discovered by Leben, Keitt, Dunchee, Scheneider and Strong⁽¹⁾⁽²⁾. It was also compared with virosin found by Nakazawa and others⁽³⁾, and antipiricullin found by Nakayama and others⁽⁴⁾. They were considered to be identical with antimycin A. Another antifungal substance, No. 720-B, produced by the same strain, was not obtained in a sufficient amount for the further studies. However, it was different from antimycin B described by Schneider, Tenner and Strong⁽⁵⁾ in the melting point.

In the present paper not only the production, purification and natures of the antifungal substances, No. 720-A and 720-B, but also characteristics of the strain are presented. In addition to the strain No. 2A-720 the authors isolated one more strain, No. 6A-369, belonging to the same Streptomyces. sp. These strains, No. 2A-720 and No. 6A-369, were compared with the virosin-producing strain. Nakazawa and others reported that among known species the virosin producing strain was resembling to Streptomyces olivochromogenus. Characters of the strains No. 2A-720 and No. 6A-369 were very resembling to the virosin-producing strain, and these strains were considered to belong to the same species. The authors, however, could not conclude that they belonged to S. olivochromogenus. Nakayama and others named their antifungal substance, antipiricullin, since it exhibited a strong inhibition to Piricularia oryzae, an organism causing a most serious disease of the rice plant. The antifungal substance, No. 720-A, was studied on their inhibitory effects on plant pathogenes, and the results were also briefly presented in this paper. The toxicity of the substance, No. 720-A, to flys was also described.