The Fas antigen is a cell surface receptor that, when engaged by Fas ligand or specific agonistic antibodies, triggers apoptosis. The effect of an agonistic monoclonal antibody to mouse Fas antigen (Fas mAb, clone J02) on the viability of cells from dispersed mouse corpora lutea (CL cultures) was tested. Cultures were prepared by enzymatic digestion of CL from day 4 -7 pseudopregnant mice. Cultures were pretreated with 0, 1, 10, 100, or 1000 U/ml murine interferon-␥ (IFN) at 72 h of culture. IFN has been shown to increase Fas antigen expression in a number of cell types. At 96 h (time zero), cultures were treated with Fas mAb or IgG. By 4 h after Fas mAb treatment, discrete homogeneous patches of cells within the cultures showed characteristic signs of apoptosis, including blebbing of cell membranes, detachment, and disappearance from the culture. CL cultures contain luteal, stromal, and endothelial cells; fibroblasts; and surface epithelial cells (OSE). Cells dying in response to Fas mAb were identified as OSE. Affected cells had the cobblestone appearance and distinct nuclei typical of epithelial cells. Unlike luteal cells, OSE did not stain with the lipophilic dye, Nile red. The cells did not stain with acetylated low density lipoprotein conjugated to the fluorescent marker octadecyl indocarbocyanine, a marker for endothelial cells and monocytes. Cells in patches stained positively for cytokeratin, a marker for epithelial cells. Fas-mediated cytotoxicity was quantified by counting the number of cells present in discrete patches of OSE 0 and 8 h after Fas mAb treatment. Fas mAb treatment had no effect in cultures pretreated with 0 or 1 U/ml IFN, but induced significant death of OSE in cultures pretreated with 10, 100, and 1000 U/ml IFN (37 Ϯ 11%, 54 Ϯ 18%, and 60 Ϯ 11%, respectively). There was no apparent effect of Fas mAb on other cell types within the CL cultures. To confirm that cells dying in response to Fas mAb were OSE, experiments were also performed on enriched cultures of OSE prepared by enzymatic digestion of the outer surface of the ovary. In enriched OSE cultures pretreated with 200 U/ml IFN, there was 44% killing in response to Fas mAb, whereas in cells not pretreated with IFN, there was no effect. T HE OVARY is covered with a single layer of epithelial cells, the ovarian surface epithelial cells (OSE). OSE on the preovulatory follicle undergo apoptosis at the time of ovulation (1, 2) and then proliferate rapidly to repair the ruptured follicle and cover the surface of the developing corpus luteum (CL) (3). OSE are important in the oncogenesis of ovarian cancer; more than 90% of ovarian cancers arise from the OSE (4). It has been postulated that the oncogenic potential of OSE is linked to the extensive proliferation of these cells that is required during each ovulatory cycle (5). Factors affecting the death and proliferation of OSE are poorly understood. Defining these factors could provide insight into the processes by which OSE are transformed into cancerous cells.The Fas antigen (CD95,...