Mutagen-sensitive mutants are collected in Drosophila melanogaster as a source of mutants defective in DNA repair mechanisms. The mutants are characterized cytogenetically in order to gain a basic understanding of the genetic control of sensitivity to mutagenic agents. The tests used in the initial characterization of these mutants include genetic and cytogenetic mapping, complementation analysis tests for sensitivity to unrelated mutagens, and tests for pleiotropic effects on related functions such as recombination. Some of these mutants may not be defective in DNA repair, but may affect metabolic (in)activation or permeability to chemical mutagens.