The accumulation of leucocytes in milk and in teat cistern tissues, and the presence of the cytokines interleukin-1 beta (IL-1 beta), tumour necrosis factor-alpha (TNF-alpha), interleukin-8 (IL-8), granulocyte-macrophage colony-stimulating factor (GM-CSF) and interferon-gamma (IFN-gamma) in milk was studied during inflammation in the lactating ovine udder induced by Staphylococcus aureus and Escherichia coli. Milk and/or teat tissue samples were taken before and four, eight and 24 hours after inoculation. Both S aureus and E coli induced a significant accumulation of leucocytes in milk, S aureus from eight hours with a peak after 24 hours, and E coli from four hours, with a peak after eight hours. After the inoculation of S aureus the numbers of subepithelial neutrophils in the teat cistern tissues were increased after four hours and had increased further by eight hours. After the inoculation of E coli, tissue neutrophil numbers peaked at four hours and were still high at eight hours. Large numbers of S aureus were recovered from milk throughout the experiments whereas the recovery of E coli diminished over time. TNF-alpha, IL-8 and IL-1 beta were detected in milk during the inflammation, and low levels of GM-CSF and IFN-gamma were also observed occasionally. Of the cytokines measured, TNF-alpha and IL-8 reached the highest concentrations, accumulating in a similar pattern to the leucocytes for both infections. IL-1 beta was detected mainly during S aureus-induced inflammation, indicating differences in the inflammatory responses elicited by the organisms.