A new antibiotic was isolated from the fermentation broth of an unclassified species of Streptomyces. The antibiotic activity against Serratia sp. and Bacillus subtilis, grown in a chemically defined minimal medium, was reversed by the addition of L-glutamine to the medium. Physical chemical characterization of the antibiotic and of the chemical and enzymatic degradation products revealed the structure to be L-(N5-phosphono)methionine-S-sulfoximinyl-L-alanyl-L-alanine. The deliberate search2~4) for antimetabolites in fermentation broths has, in three instances5~~7), resulted in the isolation of biologically active di- and tripeptides. We now wish to report the occurrence and isolation of a fourth member of this group, L-(N5-phosphono)methionine-S-sulfoximinyl-L-alanyl-L-alanine, I.** This substance is produced by an unclassified Streptomyces sp. X-13152. It inhibits the growth of Serratia sp. and Bacillus subtilis on a chemically defined minimal medium8) but addition of L-glutamine to the medium relieves this growth inhibition. Originally9) the antimetabolite was isolated by an elaborate procedure from broths containing concentrations of about 1 ^g/ml. However, a medium improvement and strain selection program increased the fermentation yield over 10-fold and the isolation procedure reported here is suitable only for the high yield broths. The details of the structure elucidation are being reported separately9) and hence are only