Microoganisms producing antibiotics have been genetically converted by earlier workers to mutants which cannot produce antibiotic without supplementation with a moiety of the antibiotic. These antibiotics include neomycin, kanamycin, paromomycin, butirosin, sisomicin, ribostamycin and novobiocin. Success has not been reported for organisms producing guanidinocyclitol antibiotics such as streptomycin. We mutagenized conidia of the streptomycin-producing Streptomyces griseus strain 7-455F3 with nitrosoguanidine at pH 7.0. Non-producers of streptomycin were visually selected by the agar-plug technique using Bacillus subtilis. We successfully isolated mutant MIT-A5 which produces no streptomycin unless streptidine is added to the agar medium. The streptidine-dependent phenotype was confirmed in submerged culture in flasks. Attempts to produce new antibiotics by feeding aminocyclitols to mutant MIT-A5 failed. However a new antibiotic (streptomutin A) was produced by supplementation with the guanidinocyclitol, 2-deoxystreptidine. We propose the term "mutational biosynthesis" for the production of new metabolites by the use of mutants blocked in the biosynthetic pathway to the secondary metabolite. We further propose the term "idiotroph" to properly describe such mutants.