Intraspecies fusion of protoplasts of two strains of Streptomyces fradiae, i.e native protoplasts of an inactive strain INA 00708 and heat inactivated protoplasts of a neomycin-producing strain ATCC 10745, and regeneration of the protoplasts of the inactive strain INA 00708 resulted in formation of clones producing neomycin and clones synthesizing antibiotics of an unknown nature differing from neomycin. All the active clones were unstable and lost their antibiotic activity in subcultures. Regeneration of the protoplasts of 4 different inactive strains of Streptomyces sp. also resulted in formation of active clones which were unstable and lost their capacity for the antibiotic synthesis after the first subculture. The data in principal indicate to the possible use of protoplast fusion and regeneration in screening of cultures producing new antibiotics among inactive strains of streptomycetes. However, the efficiency of such procedures is low since the experiments are labor-consuming and the resulting active clones are genetically unstable.