Epithelial ovarian cancer (EOC) remains a lethal disease because few specific biomarkers exist for early stage detection. Many serous EOCs arise from fluid-filled "inclusion" cysts in the ovary, lined with cells from ovarian surface epithelium (OSE). Kallikrein serine protease 4 (KLK4) is poorly expressed in OSE, but up regulated in serous EOC cell lines and tumours and is thus a potential biomarker for serous EOC. Mouse ovaries subjected to incessant ovulation form ovarian inclusion cysts which are similar histologically to human benign serous cystadenomas. We investigated Klk4 protein expression in mouse ovaries with high lifetime ovulation number, to determine if Klk4 protein was up-regulated during inclusion cyst development and whether Klk4 expression correlated with histopathology in the mouse ovary. Incessant ovulation was induced for up to 12-months by keeping Swiss Webster mice in cages divided by a screen. Breeding stock of the same age were controls. Mouse Klk4 expression was measured by immunohistochemistry with three anti-hKLK4 anti-peptide antibodies, directed to the N- terminus (Ab01), mid region (Ab03) and C-terminus (Ab05) of human KLK4. Cytoplasmic immunoreactivity was detected with Ab01 and Ab03 and nuclear immunoreactivity with Ab05, in stromal cells, luteal cells, granulosa cells of some apoptotic antral follicles and in some, but not all oocytes in primordial or primary follicles, in ovaries from incessantly ovulated mice or breeders. No immunoreactivity was detected in OSE. Cysts were observed in ovaries from both breeders and incessantly ovulated mice. Immunoreactivity with Ab01/03 was inconsistent in cyst cells, although some dysplastic cells showed basal cytoplasmic staining with Ab03. Strong nuclear staining was observed with Ab05 in the majority of cyst cells, with weaker staining in rete ovarii. We conclude from these preliminary studies that Klk4 expression is up-regulated in inclusion cysts observed in cystic mouse ovaries.