Acid hydrolysis of amidinomycin gives two Ninhydrin-positive compounds, a new amino acid, 3-aminocyclopentanecarboxylic acid, and a base, 2-amidinoethylamine. The name of cyclopentaminin is given for the new amino acid, and its characters and synthetic method are described. The structure of N-(2-amidinoethyl)-3-amino cyclopentanecarboxamide is proposed for amidinomycin.