L-2-Amino-4-pentynoic acid was isolated from a streptomycete fermentation and shown to inhibit the growth of Bacillus subtilis in a chemically defined medium. This growth inhibition was reversed by L-methionine or L-leucine. Recently several laboratories2-11) have been interested in the search for antimetabolites in fermentation broths. Such compounds are, ordinarily, not observed in conventional antibiotic screens due to the presence of reversing metabolites in the complex media commonly employed. However, by the use of chemically defined minimal media12) it is possible to detect antimetabolite activities and, by selectively supplementing the assay medium with known substances, to determine which compounds relieve the growth inhibition. In this paper we report the isolation of an antimetabolite of L-methionine and L-leucine from a fermentation broth produced by an unidentified streptomycete*. The substance which has not been reported in natural sources proved to be L-2-amino-4-pentynoic acid identical with an authentic sample prepared from synthetic racemic.