Abstract In in vitro experiments, the one-hour incubation of neutrophils from peripheral blood of 10 sheep (Polish Merino, aged 1 year) with ammonium acetate to final ammonia concentrations of 7.1, 71.0, 355.0, and 710.0 μmol/l caused no changes in cell viability. Incubation prolonged to 2 h significantly reduced cell viability at all ammonia concentrations. In contrast, incubation of neutrophils for 45 min with ammonium acetate significantly decreased their phagocytic activity against non-opsonized bacteria of Staphylococcus aureus 305 (all test concentrations) and against opsonized bacteria of S. aureus Smith strain (355.0 and 710.0 μmol/l only). In in vivo experiments, subclinical hyperammonaemia was produced in 10 sheep by continuous, three-day infusion of ammonium acetate solution into the right jugular vein. Ten control animals received an infusion of 0.9% sodium chloride. Blood was sampled from the left jugular vein before infusion, and 24, 48 and 72 h after beginning, and 48 h after the end of infusion. A significant rise in blood ammonia concentration occurred 24 and 48 h after beginning infusion. This increase in the experimental group correlated with a significant fall in phagocytic activitty of neutrophils against non-opsonized S. aureus 305 bacteria as compared to values before the infusion and to the controls. In contrast to the results of the in vitro experiment, no statistically significant influence of hyperammonaemia on neutrophil phagocytic activity against opsonized bacteria of S. aureus Smith was found in vivo.