Genetic process for the preparation of antibiotic-producer micromonospora strains

A process for preparing an antibiotic-producing Micromonospora strain having modified genetic material, which comprises preparing, following cultivation in a glycine medium, a protoplast suspension of each of two genetically different mutants of antibiotic-producing Micromonospora with lysozyme, under osmotically buffered conditions ensured by sucrose, combining the two suspensions thus obtained in the presence of polyethylene glycol, incubating at room temperature, suspending the resulting fused protoplasts in soft agar, then plating the suspended protoplasts on an agar plate containing sucrose, proline and inorganic salts, incubating the plated protoplasts for 20 to 30 days, selecting all colonies from the regenerated colonies which are sure to have genetic material different from that of said two mutants, and, prior to fusion, heat-inactivating the protoplasts of one of said mutants by heating them at 55.degree. C. for three hours, thereby to permit the genetic material of the non-heated protoplasts to be transferred into the cytoplasm of the heat-inactivated protoplasts.