A new crystalline amino acid was isolated from the fermentation broth of a Bacillus species and identified as N5-hydroxy-L-arginine. This compound possesses antimicrobial activity against several microorganisms, including Escherichia coli, which is reversed by L-arginine and related compounds. The N-hydroxy function is frequently encountered in microbial products, most commonly as part of hydroxamic acids. Naturally occurring hydroxamic acids consist of a hydroxylamine moiety generally supplied by N-hydroxyamino acids such as N5-hydroxyornithine, and an acyl portion which is usually acetyl or biogenetically derived from acetate. We now report the discovery of N5-hydroxy-L-arginine representing a new amino acid and formally resembling hydroxamic acids in that it contains an N-hydroxy function in the form of N5-hydroxyornithine, but the acyl moiety is replaced by an amidino group. N5-Hydroxy-L-arginine is produced by a microorganism isolated from soil collected in Petit Saint Vincent Island, the Grenadines, British West Indies, identified as a member of the genus Bacillus, and designated Bacillus sp. XB-13248.