We have previously shown that mouse tears are cytotoxic to the corneal keratocytes. Since tear components are derived from both lacrimal tissues and ocular surface epithelium, we sought to determine the source of the cytotoxic factors in the mouse tear fluid. Cytotoxicity to keratocytes was assessed by an ex vivo assay using an isolated eye; after treatment with test samples, segmentation and disappearance of stromal nuclei were determined by DAPI nuclear staining. Following biological tissues and fluids were examined either directly or after preincubation at 37 degrees C for 2-15 hr: extraorbital lacrimal gland (ELG), intraorbital lacrimal gland (ILG), Harderian gland, Meibomian gland, corneal epithelium, bulbar conjunctiva, palpebral conjunctiva, serum, aqueous humor, and lacrimal fluid collected from a secretory duct of ELG. Under the ex vivo assay conditions, ELG and ILG, with or without preincubation, exhibited a cytotoxic effect comparable to that of diluted tears. Lacrimal fluid collected from an ELG duct was similarly effective. These specimens triggered nuclear segmentation that is typical of apoptotic nuclei. All other specimens showed no effect on stromal nuclei under the identical conditions. In some animals, ELG was surgically removed and the tear cytotoxicity was examined in vivo. The tear cytotoxicity in these animals was lost after the surgery, indicating an involvement of ELG, but it was restored 4-24hr afterward, suggesting a compensatory role of ILG.These results suggest that ELG and ILG are the major sources of tear factors that are cytotoxic to the keratocytes in the mouse.