Interspecies fusion of protoplasts of the Streptomyces fradiae strains producing neomycin (an aminoglycoside antibiotic) and tylosin (a macrolide antibiotic) was performed with a view to isolate strains producing novel antibiotics. Fusion of the protoplasts of the neomycin- and tylosin-producing strains labelled by the resistance to monomycin and lincomycin, respectively, caused no formation of stable strains producing antibiotics differing in chromatographic mobility from the antibiotics produced by the initial strains. In fusion of the protoplasts of the unlabelled strains, heat-inactivated protoplasts of the active line of one strain (donor) and native protoplasts of the inactive line of the other strain (recipient) were used. When the neomycin-producing culture was used as a recipient the fusion led to formation of strain 195-34 producing antibiotics of the benzo(a)anthraquinone group. One of these antibiotics, i.e. antibiotic 34-I, proved to be a novel biologically active substance. After regeneration of the protoplasts of the initial strains, no stable strains producing antibiotics differing from neomycin and tylosin were isolated.