A feedback loop for the regulation of the rec/lex-mediated DNA repair system is proposed. This model was formulated from experiments on the genetic and metabolic regulation of the rate of synthesis of protein X performed in this laboratory, and from genetic data obtained in other laboratories. Protein X is proposed to prevent DNA degradation by the recBC-coded exonuclease. The model states tht: (1) The lex (or exrA in E. coli B) gene codes for a repressor. (2) This repressor binds to an operator region of DNA consisting of the tif-zab region at 51 minutes on the E. coli chromosome. (3) The operator region controls the production of several proteins involved in DNA repair, including protein X. (4) The recA gene product is required to remove the lex-coded repressor from the operator. Thre recA gene could code for an antirepressor (inducer protein or a protease) or a modifer of recBC nuclease action; (5) Low molecular weight products of DNA degradation are effectors that activate the system. (6) Protein X limits recBC nuclease action by binding to single-stranded DNA.