Abstract: The invention provides a process for the preparation of antibiotic-producer Micromonospora strains having modified genetic material, wherein a protoplast suspension is prepared with lysozyme, under osmotically buffered conditions ensured by sucose, from each culture of the two genetically marked mutants of the antibiotic-producer Micromonospora strain following cultivation in a glycine medium, both suspensions obtained are combined in the presence of polyethylene glycol, and incubated at room temperature, the resulting fused protoplasts are suspended in soft agar, then plated on an agar plate containing sucrose, prolin and inorganic salts, incubated for 20 to 30 days, and finally all mutants are selected from the regenerated colonies which are sure to have genetic material different from that of the parent strains.The process can be advantageously applied to improve productivity of antibiotic-producer strains having industrial importance.