The peptide antibiotic PI 68, isolated from a culture filtrate of Paecilomyces lilacinus (Thorn) Samson and possessing a wide antimicrobial spectrum against fungi, yeasts, and gram-positive bacteria, contained a new amino acid, (2S,4S)-2-amino-6-hydroxy-4-methyl-8-oxodecanoic acid (6), and an amine, (S)-N¹,N¹-dimethylpropane-1,2-diamine (4), along with other unusual amino acids. The structure of the peptide was determined as (I) by in-beam mass spectrometry. In this paper, we elucidate the structure of the peptide antibiotic PI 68 in detail and describe the determination of the stereochemistry of the acid and the amine components.