The effect of treatment with various intercalating dyes on the ability to produce antibiotics in Micromonospora rosaria and Micromonospora purpurea was studied. Treatment with acriflavine resulted in a high frequency loss of antibiotic productivity in both species. In M. rosaria, the loss of antibiotic-producing ability appeared to be strain-dependent. In M. purpurea, up to 90% of colonies were found to have lost gentamicin-producing ability when protoplasts were used in the test. These antibiotic-nonproducing strains were further studied. The following observations were made: (1) Unlike the producing ability, the resistance to the antibiotics is a very stable character in both species. (2) Protoplast fusion analysis indicates that rosamicin-nonproducing characteristics of MR 217-AF2 and MR 217-AF3 strains induced by the acriflavine treatment is due to chromosomal mutation or rearrangement but not to loss of a plasmid. (3) Gentamicin-nonproducing strains of M. purpurea responded differently to the supplementation of streptamine or DOS in the culture medium. When supplemented with streptamine or DOS, some of these strains regained the ability to produce antibiotic, showing that the biosynthesis of intermediate was affected in these strains.