The effect of conjunctival Mycoplasma bovoculi infection on local and systemic natural killer activity was studied in cattle. A standard in vitro assay against the mouse tumor cell line, YAC-1, was used to measure increases in effector cell activity. Local responses in the eye were investigated by examination of stained exfoliative conjunctival cells and tissue biopsies. An increase in systemic cytolytic activity was seen in infected calves, and maximal values were observed on the second day post infection (DPI). An elevation in the number of large granular lymphocytes was noted in the conjunctival samples, and maximal numbers were again observed at 2 days post infection. It is proposed that natural killer cell activity may play a role in the initial host responses of cattle to M. bovoculi infection. © 1993.