[Nystatin biosynthesis by inactive mutants of Act. noursei when cultured together]

Inactive mutants of the nystatin-producing organism Act. noursei were selected. When 2 of them were cultivated together, nystatin biosynthesis was restored. The substance or substances restoring the antibiotic biosynthesis was excreted into the culture broth. It had non-protein nature and was stable on storage. Reduction of the nystatin biosynthetic chain proceeded without changes in the cultivation conditions.