A new antibiotic, l-(^-hydroxyphenyl)-2,3-diisocyano-4-(^-methoxyphenyl)-buta-l,3-diene, was isolated from the mycelium of Dichotomomyces albus Saito in crystalline form by utilizing the paper disc-agar diffusion plaque inhibition method. The antibiotic inhibits the plaque formation of Newcastle disease virus infected on primary chick embryo fibroblast cell monolayer. It is active against some gram-positive and gram-negative bacteria. Through antiviral screening using Herrmann's paper disc-agar diffusion method, an acetone extract of the mycelium showed antiviral activity against Newcastle disease virus strain Miyadera with some cytotoxicity. The active principle was isolated via silica gel column chromatography, and structural studies identified it as l-(^>-hydroxy phenyl)-2,3-diisocyano-4-(^-methoxyphenyl)-buta-l ,3-diene (xanthocillin X monomethyl ether). Biological properties were investigated, including antiviral activity in twofold serial dilution experiments starting from 16.5 mg/ml showing effective plaque inhibition, and antimicrobial spectrum against organisms like Staphylococcus aureus, Bacillus subtilis, Escherichia coli, etc. The antibiotic showed no definite melting point, turning dark yellow at 120°C and decomposing at 166°C. It had no cytotoxicity in agar diffusion assays at tested doses but showed cytotoxicity at 3~6 mcg/ml in tube cultures, with an LD50 of approximately 0.3 mcg/ml on HeLa cells. It was less effective against vaccinia and herpes simplex viruses, with suppression of cytopathic effect only at slightly toxic doses.