The cells present in the conjunctival sacs of 142 healthy sheep and 87 sheep with ovine keratoconjunctivitis were examined. The most numerous cells in the healthy conjunctival sacs were epithelial cells and occasionally lymphocytes; neutrophils were rarely present. Ovine keratoconjunctivitis was characterised by a rapid onset of acute inflammation of the conjunctiva, followed by hyperaemia of the sclera and pannus, and opacity of the cornea. The most numerous cells in the acute phase were neutrophils, and the presence of plasma cells was also suspected. The use of cytological methods to aid the detection of Mycoplasma conjunctivae, the organism most commonly isolated from cases of ovine keratoconjunctivitis in Britain are discussed.