Antibiotic complex 4205, a basic polypeptide, was isolated from a nonsporulating bacillus. It was separated into two components, A and B, each composed of 2,4-diaminobutyric acid, isoleucine, leucine, phenylalanine, serine, and valine, but in different proportions, namely, 4:1:2:1:1:1 and 5:1:3:1:2:2, respectively. There was also an ether-soluble moiety in each that had the characteristics of a fatty acid. This antibiotic differed chemically from other known antibiotics. Both A and B were effective against gram-positive and gram-negative bacteria, cytotoxic for various tissue cell lines, highly toxic for mice by intraperitoneal injection, and toxic for chick embryos by the amniotic and allantoic routes, but not by yolk-sac inoculation. Antibiotic, in amounts sublethal by the allantoic route, inhibited the multiplication of 10 to 100 minimal infective doses of influenza type A, PR8, in embryonated eggs.