A generally similar clinical response was observed in six lactating Holstein-Friesian cows after intramammary inoculation with approximately 107 colony-forming units of Streptococcus uberis. Increased concentrations of serum amyloid A (SAA) were measured in both milk and serum taken 6 and 11 h after inoculation, respectively. In contrast, increased concentrations of haptoglobin were detected after 10 h of infection, in milk only. In the blood, tumour necrosis factor-alpha (TFN-alpha) was detected (0.503 ng/ml) in only one animal, at the time of euthanasia (10 h after infection). Interferon-gamma (IFN-gamma), like haptoglobin, was not detected in blood. Parallel to the development of inflammation and influx of inflammatory cells into the udder tissue, a marked decrease in the number of monocytes and neutrophils in blood was observed. Bacteria were found both intracellularly (macrophages and neutrophils) and within the lumen of ducts and alveoli. Lesions developed progressively in an ascending manner and became widespread throughout the mammary gland in less than 8 h. The parallel development of inflammation and increased concentrations of SAA and haptoglobin in milk points to these acute phase proteins as potential diagnostic markers for the early detection of S. uberis-associated mastitis.