A number of years ago during the course of a program designed to uncover Gram-negative antibiotics, a weakly active material was produced by fermentations of an unidentified Streptomycete (Lederle culture BM726). This strongly basic (Sakaguchi positive), water-soluble antibiotic [a]'D5 -128 -2° (c 0.7, H₂O), was isolated from mash filtrate using a dextran weak cation exchanger followed by elution with a salt gradient. The active material was then recovered from the salt solutions by use of granular carbon. A recent re-examination of the spectral data along with some new mass spectral results (fast atom bombardment mass spectrometry-FABMS) have enabled us to unravel the structure as that of I, a dipeptide consisting of L-arginine and a novel cyclopentenonyl glycine.