Many bacterial plasmids have the effect of increasing the ultraviolet (u.v.) resistance of host cells that harbour them, apparently by an error-prone repair mechanism that leads to a high level of mutation amongst the survivors. These plasmid systems are apparently analogues of the Escherichia coli umuD/C operon, which is absolutely required in this organism for mutation induced by u.v. light and by many chemical mutagens. This article reviews the extensive and sometimes conflicting literature relating to this phenomenon, and describes the further characterization of one such plasmid system, the imp (I group mutation and protection) operon of the I1 group plasmid TP110. It is demonstrated that each of the protection mutation systems well characterized to date shows a similar genetic arrangement, and that significant homology can be detected at the amino acid level between the proteins encoded by these different systems.