It was found that (−)rugulosin, an antibiotic isolated from Myrothecium verucaria, had a potent anti‐phage effect on RNA phages (MS2, GA and Qβ) and DNA phages (δA and T4). The effect was almost independent of the host bacterial strains used. By a detailed investigation using MS2, it was revealed that the antibiotic did not affect the free phage or host bacterium alone but inhibited phage multiplication, and the degree of the inhibition depended on the multiplicity of infection. The inhibition was not mainly due to a drop in the burst size but rather was due to a decrease of the phage‐producing cells during the early stages of phage infection and replication.

The mechanism of anti‐phage action of (—)rugulosin was investigated mainly using phage MS2. In the presence of the antibiotic, phage adsorption was not inhibited and phage–bacterium complexes did not remain in an antiserum‐sensitive state. However, phage–bacterium complexes temporarily remained in a shear‐sensitive state during the time immediately following phage infection and phage RNA penetrating into the host bacteria was decreased. The antibiotic also acted on preformed antiserum‐ and shear‐resistant infective centers and inhibited the in vitro RNA synthesis by Qβ replicase. These results indicate that the mechanim of action of the antibiotic consists of at least 2 parts, i.e., the inhibition of the penetration of phage RNA into the host bacteria and some early intracellular reaction(s) for phage multiplication.

(-)Rugulosin showed an antiinfluenzal effect in the allantoic cavity of chicken eggs as well as in cultured chorioallantois, proving itself to be more potent than 1-adamantanamine.Also, ( -)rugulosin, when administered by inhalation, was as effective for protection of mice against aerosol influenzal infection as orally administered 1-adamantanamine.The effect of ( -)rugulosin seemed to be mainly due to direct inactivation of influenza virus.