We have isolated spontaneous temperature-resistant revertants of a temperature-sensitive mutation (rpoD800) in the sigma subunit of E. coli K12 RNA polymerase. These revertants still contained the rpoD800 allele. They were mucoid, and sensitive to ultraviolet light and the radiomimetic agent nitrofurantoin, which are characteristics of lon mutants. One revertant, Tr29, was mapped to the lon region of the chromosome. Lon- rpoD800 double mutants were constructed, and were phenotypically indistinguishable from the spontaneous temperature-resistant revertant. It is the degradation-deficient property of lon mutants that is responsible for the suppression of the temperature-sensitive phenotype. We show that the rpoD800 sigma polypeptide is a substrate for the ion proteolytic system, and that mutations in lon decrease the rate of mutant sigma degradation. The rate of synthesis of mutant sigma was also affected in lon- strains. The net effect of lon-mutations was to increase the concentration of mutant sigma. We conclude that the temperature-sensitive phenotype results from insufficient concentration, rather than altered function, of the mutant protein.