To identify the nature and origin of spontaneous mutability we developed a screening procedure suitable to isolate antimutators showing a lower error rate than 10(-10) per base per replication. Among about 500,000 mutagenized colonies we found 20 mutants showing a reduced spontaneous mutability. These antimutators can be subdivided into three groups: (i) Mutants in which the level of spontaneous mutability is reduced due to an increase in efficiency of the error correcting mechanism (amu4). (ii) Mutants which are deficient in several pathways of DNA repair. This finding supports the hypothesis that much spontaneous mutability is due to error-prone repair (amu59, amu47, amu50, amu62, amu43, amu38). (iii) Mutants in which the antimutator effect seems to be the result of an auxotrophy such as Pur- (amu17), Thr- (amu1, amu28) and Ser- (amu31). This finding might support the hypothesis that metabolically induced lesions are important in spontaneous mutagenesis. Eleven of these antimutators were mapped at ten bacterial loci in the following positions: amu31 (2 min); amu4 (4 min); amu62 (82 min); amu47 (85 min); amu59 (86 min); amu17 (89 min); amu50 (95 min); amu1/amu28 (100 min); amu38 (23-27 min) and amu43 (74-81 min).