A hypothesis was proposed several years ago that Escherichia coli posses an inducible DNA repair system ("SOS repair") which is also responsible for induced mutagenesis. Some characteristics of the SOS repair are (1) it is induced or activated following damage to DNA, (2) it requires do novo protein synthesis, (3) It requires several genetic functions of which the best-studied are recA+ and lex+ of E. coli, and (4) the physiological and genetic requirements for the expression of SOS repair are suspiciously similar to those necessary for the prophage induction. The SOS repair hypothesis has already served as the working hypothesis for many experiments, some of which are briefly reviewed. Also, some speculations are presented to stimulate further discussions and experimental tests.