In our screening for antiviral antibiotics using the agar diffusion-plaque inhibition method, a new antibiotic was obtained in crystalline form from the filter cake of the fermented broth of Dichotomomyces albus Saito. The structure of the antibiotic was determined to be l-(p-hydroxyphenyl)-2,3-diisocyano-4-(p-methoxyphenyl)-buta-l,3-diene from the interpretation of the ultraviolet absorption, infrared absorption, nuclear magnetic resonance and mass spectra. A new antiviral antibiotic was found in the filter cake of the fermented broth of Dichotomomyces albus Saito by utilizing the paper disc-agar diffusion-plaque inhibition method. The antibiotic effectively inhibits plaque formation of Newcastle disease virus strain Miyadera (NDV) infected on primary chick embryo fibroblast cell monolayer (GEF) and the growth of some pathogenic bacteria, such as Shigella flexneri and Staphylococcus aureus. The structure of the antibiotic was studied by examining the ultraviolet (UV), infrared absorption (IR), mass and nuclear magnetic resonance (NMR) spectra of the antibiotic. It is remarkable that the antibiotic has an isocyano grouping in the molecule. The structure proposed is l-(p-hydroxyphenyl)-2,3-diisocyano-4-(p-methoxyphenyl)-buta-l,3-diene on the basis of the spectral evidences and from IR comparison with known antibiotics with isocyano groupings, xanthocillin X (XX) and its dimethylated derivative.