Natural killer (NK) cells and acquired cell-mediated immunity effector cells (delayed type hypersensitivity (DTH) and cytotoxic T lymphocytes (CTL)) have been reported to play a vital role in the defence of the host against tumour and viral infections in locations other than the eye. A vigorous cellular inflammatory response to viral infections of the cornea, however, with the attendant damage to the corneal clarity, has obvious evolutionary disadvantages, and a substantial body of evidence indicates that in animals (e.g. mice) which are highly susceptible to inflammatory destruction of the cornea following corneal encounter with herpes simplex virus, it is the animal's immunological/inflammatory response which is responsible for the corneal damage. We examined the role of natural killer cells in the development of herpes stromal keratitis (HSK) in NK-deficient (C57BL/6J-bgj (beige)) mice and their NK-competent (C57BL/6J (black) relatives. The beige (NK-deficient) mice were just as resistant to HSK as were the black mice. We also studied the effects of NK cell depletion of BALB/c Igh-1 disparate congenic mice. C.AL-20 (Igh-1d) mice are ordinarily highly susceptible to necrotising HSK. In vivo NK-cell depletion in these mice significantly decreased the incidence and severity of HSK in these animals (p < 0.0005). Corneas from untreated C.AL-20 mice contained T cells, macrophages and NK cells. The corneal infiltrate from NK-depleted C.AL-20 mice consisted of T cells and macrophages but no NK cells. These data indicate that NK cells are participants in the development of HSK in the murine model of this disease.