An efficient and relatively simple procedure for Micromonospora melanosporea protoplast preparation and transformation is described. Transformation of M. melanosporea protoplast by the Streptomyces plasmid pIJ702 was optimized by altering parameters affecting the formation, regeneration, and transformation of protoplasts. Improvement of regeneration medium resulted in relatively quick growth of transformants (only 7 days). As a result of these experiments we describe a new transformation method that has routinely yielded 10(6) transformants/mu-g plasmid DNA.