Mammals produce sperm or oocytes depending on their sex; however, newborn MRL/MpJ (MRL) male mice produce oocytes within their testes. We previously reported that one of the genes responsible for this phenotype is present on the MRL-type Y chromosome (Y(MRL)), and that multiple genes, probably autosomal, are also required for the development of this phenotype. In this study we focused on the autosomal genes and examined their relationship with this phenotype by analyzing the progeny from crosses between MRL mice and other strains. We first observed the male F1 progeny from the crosses between female A/J, C57BL/6 (B6), BALB/c, C3H/He, or DBA/2 mice and male MRL mice, and two consomic strains, male B6-Y(MRL) and MRL-Y(B6). Testicular oocytes that were morphologically similar to those of MRL mice were detected in all mouse strains except BALBMRLF1; however, the incidence of testicular oocytes was significantly lower than that in MRL mice. The appearance of testicular oocytes in MRL-Y(B6) mice indicates that this phenotype is strongly affected by genomic factors present on autosomes, and that there is at least one other causative gene on the MRL-type autosomes (MRL testicular oocyte production, mtop) other than that on Y(MRL). Furthermore, a quantitative trait locus (QTL) analysis using N2 backcross progeny from crosses between female MRLB6F1 and male MRL mice revealed the presence of susceptibility loci for the appearance of testicular oocytes at 8-17 cM on Chr 15. These findings demonstrate that the appearance of testicular oocytes is regulated by the genetic factors on Chr 15 and on Y(MRL).