Deamination of bases is regarded as the major factor for mutation in DNA and it will induce transition mutation if the products of deamination have not heen repaired. Now a new sensitive genetic assay to determine quanti-tatively deamination of bases in DNA special site has been introduced to understand the relationship between DNA structure and its chemical activities. The assay is based on reversion of a mutant of CCC proline coding sequence which is located in Lac Z α gene of bacteriophage M13mp2 from a colorless to blue plaque phenotype. This approach is highly sensitive; deamination of a single cytosine residue in 10⁵ M13mp2 DNA molecules can be detected. Besides, the kinetic rate constant of the reaction and the activation energy of deamination can be calculated.