The mei-9(a) mutant of Drosophila melanogaster , which reduces meiotic recombination in females (Baker and Carpenter 1972), is deficient in the excision of UV-induced pyrimidine dimers in both sexes. Assays were performed in primary cultures and established cell lines derived from embryos. An endonuclease preparation from M. luteus , which is specific for pyrimidine dimers, was employed to monitor UV-induced dimers in cellular DNA. The rate of disappearance of endonuclease-sensitive sites from DNA of control cells is 10-20 times faster than that from mei-9(a) cells. The mutant mei-218, which is also deficient in meiotic recombination, removes nuclease-sensitive sites at control rates. The mei-9(a) cells exhibit control levels of photorepair, postreplication repair and repair of single strand breaks. In mei-9 cells DNA synthesis and possibly postreplication repair are weakly sensitive to caffeine. Larvae which are hemizygous for either of the two mutants that define the mei-9 locus are hypersensitive to killing by the mutagens methyl methanesulfonate, nitrogen mustard and 2-acetylaminofluorene. Larvae hemizygous for the mei-218 mutant are insensitive to each of these reagents. These data demonstrate that the mei-9 locus is active in DNA repair of somatic cells. Thus functions involved in meiotic recombination are also active in DNA repair in this higher eukaryote. The results are consistent with the earlier suggestions (Baker and Carpenter 1972; Carpenter and Sandler 1974) that the mei-9 locus functions in the exchange events of meiosis. The mei-218 mutation behaves differently in genetic tests and our data suggest its function may be restricted to meiosis. These studies demonstrate that currently recognized modes of DNA repair can be efficiently detected in primary cell cultures derived from Drosophila embryos.