The purpose of this work was to study the action of acridine orange and mitomycin C on the production of mutants of Streptomyces hygroscopicus, strain 0681. The mutagen activity of mitomycin C was shown to be high. The vegetative culture grown for 24 hours produced more mutants under the action of mitomycin C than spores did. Both acridine orange and mitomycin C caused the production of mutants in strain 0681. The morphology of colonies in these mutants was modified, the synthesis of hygromycin B was completely or partly blocked, and the composition of fatty acids changed quantitatively and qualitatively. Mutants induced by mitomycin C were highly active in the production of the complex of hygrolytin proteolytic enzymes.