Strains of Escherichia coli K12 heterozygous for the R100-1 tetracycline resistance region were constructed. They carried the wild-type Tetr genes in the chromosome and single site Tets mutations on plasmids. Some heterozygotes could not express tetracycline resistance fully after induction. The mutant tet allele was thus partially dominant. When heterozygotes carrying the dominant tet mutant were plated on agar containing 20 mg/ml tetracycline, mutants which grew normally occurred at a frequency of 1-4 X 10(-4). Analysis of these dominance relief mutants showed that in 53/56 isolates the dominant tet allele was lost forming either Tra+ or Tra- deletion mutants of the plasmid. The mutation frequency was not affected either by the host chromosomal recA mutation or by the temperature of growth of the culture.