Potentially mutagenic lesions occur repeatedly in the DNA of living organisms. Only the presence of exceptionally active DNA repair systems prevents rapid loss of genetic fidelity. Recent publications cited in this short review demonstrate that bacteria continue to provide valuable insights into DNA-repair mechanisms and mutagenesis. Bacteria also serve as a principal short-term screen for detection and analysis of environmental agents responsible for increasing the endogenous load of mutational lesions in mammalian DNA. These aspects of research utilizing bacteria are important, for they clearly have applicability to the cancer problem (see [ l ] for an excellent review). Additionally, mutagens may contribute substantially in other ways to the demise of human health, for example by induction of "benign," "preneoplastic" foci that may comprise a significant component of aging [ 2 ] . In this article I review a few highlights of current research that have attracted my attention. Because my description is highly abridged, the reader is referred to the literature cited for details and for references to the longer history that underlies each development mentioned. Kimball [ 3 ] and Hanawalt et a1 [4] have recently supplied useful and extensive surveys of the field.