In lpr mice the insertion of an early transposable element (ETn) into intron 2 of the Fas gene, which mediates apoptosis, causes the development of massive lymphadenopathy, splenomegaly and autoimmune disease. In the present study we investigated the influence of this mutation on ovarian development of lpr mice. By means of in situ hybridisation, the expression of Fas mRNA was detected at the same levels in the ovarian cells of MRL/MpJ-lpr/lpr (MRL/lpr) mice as in those of MRL/MpJ- +/+ (MRL/+) mice. However, indirect immunofluorescence (IIF) staining with anti-Fas monoclonal antibody (mAb) on the membrane of follicle and egg of MRL/lpr mice was significantly weaker than that of MRL/+ mice. Furthermore, the expression level of Fas protein at the 45 kDa band from ovarian cell lysates of MRL/lpr mice was much lower than that of MRL/+ mice. The co-incubation of Spodoptera frugiperda (Sf9)-Fas ligand (L) cells with eggs of MRL/+ mice resulted in apoptosis of eggs, as detected by the terminal deoxynucleotide transferase mediated dUPT-nick end labelled (TUNEL) method. In contrast the co-incubation of Sf9-FasL cells with eggs of MRL/lpr mice did not generate apoptosis in eggs. Following intraperitoneal administration of anti-Fas mAb into both types of mice, most oocytes, a proportion of granulosa cells in the ovary and hepatocytes in liver of MRL/+ mice were positively stained by the TUNEL method, corresponding to the appearance of DNA fragmented ladders by DNA fragmentation assay, while negative signals were obtained in those cells of MRL/lpr mice. As the mice aged, the ovarian size of MRL/lpr mice was found to be much larger than that of MRL/+ mice due to the increased number of ovarian follicles. Therefore, the ovarian adenopathy in MRL/lpr mice was strongly suggested to be caused by the dysfunction of Fas antigen in the ovary.