In the current studies, flow cytometric methods were used to demonstrate that heat shock protein (hsp) 70 is constitutively expressed in ovine and bovine leukocytes but that the level of expression varies considerably between different leukocyte types and between species.We also show that expression of hsp70 is upregulated in response to an 'in vitro' heat shock treatment. The optimal temperature for heat shock of leukocytes from sheep and cattle is 43.5°C. In sheep and cattle, the relative susceptibility of leukocyte type to upregulation of hsp70 expression, as assayed as percent positive cells, by 'in vitro' heat shock was cell type specific. Best results were obtained from fresh samples; after storage at room temperature for 24 h upregulation was highly variable between animals and less than in fresh samples. These studies demonstrate that evaluation of leukocyte hsp70 expression by flow cytometry is a robust, reproducible method for use in the evaluation of cellular stress responses in cattle and sheep. The application of the methods described may be a valuable tool in assessing 'in vivo' stress responses in livestock species.