<jats:p> Butirosin, a new water-soluble aminoglycosidic antibiotic complex, active against gram-positive and gram-negative bacteria, has been isolated from fermentation filtrates of <jats:italic>Bacillus circulans</jats:italic> by ion exchange. The complex has been resolved into isomeric A and B components, C <jats:sub>21</jats:sub> H <jats:sub>41</jats:sub> N <jats:sub>5</jats:sub> O <jats:sub>12</jats:sub> , by means of Dowex 1 × 1 or Dowex 1 × 2 chromatography. Properties of the free bases and the <jats:italic>N, N′, N″, N‴</jats:italic> -tetraacetyl derivatives of butirosin A and B are given.