Roseothricin (H-277) is a streptothricin-like antibiotic obtained from Streptomyces roseochromogenus. Though fairly toxic, it possesses high antibacterial activity. The hydrolysis of roseothricin hydrochloride with 20% hydrochloric acid in a sealed tube for 48 hours at 100' gave two ninhydrin-positive products, which were separated by solubility differences of their picrates. One was β,ε-diaminohexanoic acid, an amino acid also present in streptothricin, streptolin, and viomycin, and characterized by American workers. The other substance, for which the trivial name of "roseonine" is designated, has now been shown to be 2-amino-4(or 5)-(1-carboxy-1-hydroxy-2-amino)-ethyl-2-imidazoline (I).