The LL-D05139 complex, containing LL-D05139β and azaserine, was recovered from the fermentation filtrate of Glycomyces harbinensis (NRRL 15337). A chemically defined medium was developed which favored the production of LL-D05139β. Antibiotic LL-D05139β was isolated from the fermentation filtrate by adsorption on granular carbon and further purified by chromatography on microcrystalline cellulose. Acid hydrolysis of LL-D05139β gave one molar equivalent each of alanine and serine. Both amino acids were found to have the L-configuration by GC analysis on a chiral column and alanine was assigned to be the N-terminal amino acid by Edman degradation. This information coupled with IR, UV, ¹H NMR, ¹³C NMR and MS spectral data allowed us to assign the structure of LL-D05139β as alanylazaserine. LL-D05139β demonstrated greater antibacterial and biochemical induction assay activities than azaserine. The two drugs showed similar antitumor activities.