repro23 is an autosomal recessive mutation of the mouse generated by the N-ethyl-N-nitrosourea (ENU)-induced mutagenesis program at The Jackson Laboratory. The repro23/repro23 homozygous mouse shows male-specific infertility caused by defective spermatogenesis. In the present study, we investigated the testicular pathology of the affected mouse and performed linkage analysis to determine the chromosomal localization of the repro23 locus. Histological examination of the affected testis showed that the seminiferous epithelium of the repro23/repro23 mice contained spermatogonia and early stage spermatocytes, but no spermatids or spermatozoa. Immunohistochemical staining for Hsc70t, a spermatid specific protein, confirmed the absence of elongating spermatids. These findings indicated interruption of the spermatogenesis during meiosis in the repro23/repro23 mouse. By linkage analysis using 137 affected mice of F₂ progeny obtained from crosses between repro23/repro23 female and JF1/Ms (+/+) male mice, the repro23 locus was mapped to 2.2-Mb region of mouse chromosome 7. Although this region contains several potential candidate genes for the repro23 mutation, no gene already identified as a cause of defective speramatogenesis was in this region. Therefore, the gene responsible for the repro23 mutation is suggested to be a novel gene which plays an essential role in mammalian spermatogenesis.